Based on accuracy in our guns, Taylor and I have always used .50 cal as the 'changing' calibre from 3F to 2F. We've found when loaded to the same speed at 3F, 2F gives virtually identical or better accuracy - perhaps due to lower shot to shot velocity variations.
When we were starting out in BP shooting, we quickly found that 2f fouled less for us so we used it in all calibres from .36 on up. That was with the powders available in the 70's. Today, we see no difference in fouling between 3f and 2F due to the ball and patch combinations we currently use and have been using now for 40 years
. No powder fouls just shoot and shoot and shoot - no buildup - ever.
Currently, I am shooting only 3f in my .32, however I do have accuracy loads in both .40 and .45 with 2f powder, so it matters not which I have on hand. I know dang well if I went up 5 to 10gr. in charge weight, I'd have a new accuracy load for 2F in the .32 as well.
That's the way it works for those small calibres. 5 to 10gr. increase for 2F to match the 3F ballistics - speed and accuracy. Always keep in mind, that 2f produces less pressure than 3F at any given velocity. The lower the powder's developed pressure, the less 'pressure' on the patch's integrity. That means for those with what we call loose combinations, those using patches from .010" to .015", 2F will probalby shoot better as it won't be as prone to burning out your patch, which destroys accuracy. Something to bear in mind.
The Ogre can fill us or you in on the apparent powder mix that was used 'back then'. The granulations used actually being a mix of what we today call 3f and 2F. It is quite an interesting ride. Now, if I mistakenly mix a can of powder, I really don't care - reduce the load to the middle of the normal difference between granulations and go shooting. Come to think on that, if that's all that was available - a mix of 3f and 2F, I know I could make it shoot.
I firmly believe it matters not what you use, either 2F or 3F as long as you find it's most accurate load - use a tight patch, get out and shoot - try to wear that barrel out.