Regarding the amount of supply, I look at how much I spent last turn. (Press the 'E' key).
Depending on how many unit's I'm building, add a bit to that.
And look a how much I have 'in the bank'. each factory builds 10 supply, so with
that info I can determine how much supply i need to build.
consider how much you'll need for building forts/depots
I like to keep some extra for emergencies.
(perhaps you already know the above, and are talking about stuff like the comment below,
which I have made elsewhere)
As the union, I don't like to build a bunch of artillery I can't command when using the Corps commander rule,so might as well quite a few factories. I build around 15 on the 1st turn. (actually I could build more, but don't really need to right away) This may be a mistake, I end up with plenty of supply even when building boats at every opportunity. I could build more artillery and use the extras in defense. I hate getting my guns captured for lack of command. I'd rather get good infantry commanders than the lousy artillery commanders until I can command all the infantry i need to.