One other thing worth mentioning:
Things that greatly increase "production", "manufacturing", or, more obviously, social and military production, will increase the amount of spending that 100% spending indicates. So, for instance, if you pick the political party that grants +20% to social and military production, remember that at each new form of government, your social and military spending caps increase.
So the overall point is, make mental note of what your spending level is, in BCs. Then if in subsequent turns you need to decrease the %-age, note how many BCs you are now spending. You may be spending a smaller %-age, but it's of a larger whole, and you may still be spending more than before, measured in BCs.