The interplanetary reserve is an important resource to manage and must be managed alongside everything else.
There are two ways to build the reserve, either through siphoning off a percentage of resources from each planet in the empire or by building factories on planets that already have the maximum number of factories.
In the first case, you can choose how much you want to gain per turn in the reserve. Every click will display how much you will gain at that percentage.
If you choose a planet and set it to build factories after it is already full you can't see how much you will gain per turn, it will just show up in the reserve. You can calculate it by watching the reserve for two turns and doing some subtracting, but that is the best you can do.
The important thing to keep in mind is that all money that goes from a planet into the reserve is cut in half. To gain 1000 in the reserve you have to take 2000 from somewhere else.
That is the price you pay for being able to soup up the output on planets you choose by double.
The best way to spend these resources are to (1) pour them into your newest planets to get them operational as quickly as possible by doubling their output every turn till the money is spent, or (2) to pour them into planets that can produce or research very quickly.
Artifacts worlds, Orion, and Ultra Rich planets are the worlds you want to target with the money for #2.
Note, if you pour it into a Rich world, you will double your production, you lose half putting the resources into the reserve, though, so you just break even like this, you don't actually gain anything this way. It is only really useful to do this if you want to localize your ship construction at the rich worlds for some reason.
Ultra rich, on the other hand, always comes out ahead. With ultra rich worlds you can tell the ultra rich world to pour everything into the reserve and then spend the whole reserve on beefing up the ultra rich planet's production. You get a net gain into the reserve this way every turn with no losses. This is an annoying way to get ahead, but it works.
Artifacts worlds and the orion, though, should always be pumped as much as they can be if you are trying to use research to get ahead, which is most of the time. The only reason not to keep these producing at double is if you really can't be bothered to actually go through the motions and do it. It makes gameplay more annoying, but its a very efficient way to get ahead.
That being said, the major reason to play Master of Orion 1 rather than 2 or 3 is because you like simplicity and because you don't want the life to be sucked out of you by the gameplay. Orion 2 and certainly Orion 3 have a lot of the fun taken out of them by the added complexity. If you like complexity, you are probably playing those games anyway.
With Orion 1, you are playing a very simple and easy to manage game where the tactics and research takes a front seat and the micromanaging takes more of a back seat in terms of gameplay.
The added micromanaging isn't that bad, but most players will skip it anyway. The advantage is there, but usually it is only 1% or 2% in terms of your entire empire's production and research. If you want to do it, do it; and if not, then don't.
Note, any overspending on factories goes straight to the reserves. If you put everything into factory construction and it happens that the last turn only needs 1 factory constructed when you are putting enough resources into production to make 100 factories, the resources for those extra 99 factories will go straight to reserves. It won't tell you it is doing this, it will just do it.
If you see MAX listed during factory construction and you don't want the extra resources cut in half and put into the reserves, then take down the factory slider till it is just high enough to say MAX and put the rest in research or missile defense or something else. This will keep you from losing half of those production points.
Why Reserve?[edit]
Given the loss of 50% of your production, why would you even want to use the reserve at all?
Consider that you have a well-developed planet in your back lines, and a new world on your frontier. How can your well-developed planet help defend the new world?
- It could build a ship and send it over.
- It could put its production into the reserve and the new world can take it and build bases.
The advantage of the reserve is that a ship takes time to travel from the back line well-developed planet, whereas the reserve magically teleports across dozens of parsecs to the new world. And bases are very price-efficient --- they get a lot of shielding once you get any kind of Class Whatever Planetary Shield, and have an infinite supply of missiles. So if you can build a planetary shield and some bases using reserve funds, you can get a defense of your frontier world faster than if you had to build ships and send them over.
Another advantage of the reserve is that it allows the resources to "time travel". Suppose your "back line" planet is actually a border with a staunch ally who is at Unity with you, so you did not put up any serious defenses at that planet. Suddenly your ambassador tries to assassinate the ally's emperor and the ally is now at Feud and sending over fleets to your formerly-back-line planet. Now it is well-developed and can build bases, but you've also been exchanging techs with your ally and they have similar economics and tech as you (because they were, say, Pacifistic or Honorable allies and you've also been exchanging or tributing economic techs to them as well to build them up), so by itself even those bases might still be toast. If you built up your reserve in previous turns, you can now pour it back into the same planet and build up bases at twice the speed, which can probably toast the incoming enemy fleet. If instead you made ships, those ships might be en route to, or even at, other planets on a different frontier, and might not reach your formerly-back-line planet in time to save them. If you built up bases, well, the chances of a back line planet having to fight are slim (the assassination event or a successful spy framing is fairly rare, but very annoying when it comes up against a powerful ally), and the resources you put into base-building was essentially wasted in something that will not actually be used. Thus, by using the reserve, you can put spare resources now into the reserve, for spending on an unexpected future event.
Thus, the 50% "tax" of the reserve is often worth it; the reserve buys you flexibility (i.e. it warps the space-time continuum) in exchange for this 50% inefficiency.
Now consider if you then move on to the offensive. A good way to prepare for an offense is to talk to other AIs and ask them to also declare war on your target, so that the enemy's forces are split up defending at multiple points. This works better if the other AIs like you, which can be facilitated by tributing them technology and BCs stored in your reserve. Without any BCs, you might not have any technologies that the AIs may appreciate, so BCs in the reserve are also more flexible, since they are always accepted.