I was taught by a NMLRA Champion this general rule of thumb:

Fg Cannon powder
FFg Larger bore rifles and pistols (above .40 cal.) and shotguns (12 ga. and 16 ga.)
FFFg Smaller bore rifles (under .40 cal.) and pistols
FFFg Flintlock priming only

I have stuck pretty closely by this and had great success. I have experimented a bit with the dividing line between FFg and FFFg and find that to be forgiving in certain cases. Keep in mind that I was always striving for the most accurate loads in percussion patched roundball rifles.

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