What is the national stockpile slider responsible for? Is it "upkeep" for the goods that I've already bought on the world market, or is it showing how much it costs to buy the goods on the world market in the first place? Should I just set it to 0 at peacetime?
There are requirement for...
your Military Supply - This provides new Small Arms and other Goods they need to replace old or worn out equipment. Without this replenishment of equipment, your units will become less effective. Only half as much is required in maintenance during peacetime.
and also National Stockpile - If you met the target amount, no more money need unless you use them...and remember that military supply like Small Arms and other Goods are also elements of national stockpile.
Now, the point. Actually there are two meanings of the slider.
1. % Budget of Military Supply
2. 'Buying speed' of your National Stockpile up to the target amount.
First, about military supply, if you set 100% of national stockpile slider, they will get same amount of your army's consumption from national stockpile and your national stockpile manager(?) will order exact same amount to the world market because they have to maintain their target amount.
Second, about national stockpile, if you manage some kinds of national stockpile yourself(100 of steel for railroads someday for example), they will buy up and maintain 100 at first. And if you construct railroad(-100 steel), and if your national stockpile slider setting is 100%, you will order immediately 100 steel to the world market. guess what? you need very much money at once. You could even bankrupt because of huge amount of minus budget. So when you met your target amount of national stockpile, you can reduce slider(buying speed of national stockpile). Even if you construct something, you will buy more slowly, so you don't spike world price of steel and also guarantee your healthy balance sheet.