FFg to FFg ?
 

If I run out of 2F and have to use up some 3F how much less would I use ?
I load 70 grains of 2F in my Flintlock ?
Sorry the heading is wrong ! [Wife was taking to me]

I used to us FF in my flintlock (.54) years ago and used 100grs...When I switched to FFF I worked up a load again and about 80grs was the sweet spot...By moving to FFF you will have a cleaner burn and less residue in the barrel...It's hard to guess how much your gun would prefer without shooting...Assuming it's a .45-.50 or .54 anywhere from 70-90grs is plenty for deer...

Generally load about 10% less powder if switching from Goex 2F powder to Goex 3F powder.

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Originally posted by Zonie, info. obtained from the Lyman Black Powder Handbook & Loading Manual

I used 3 f exclusively in my 50 cal flintlock, 90 gns with a Speer round ball. Worked great.

Powder and load
 

Use Pyrodex. Found using "P" in everything works very well. Was getting bit of hesitation with 3f. Burns a lot cleaner too. I worked out a formula that makes figuring powder load work very well with rifles.
3.1416 X Radius sq X barrel length X 15,72 = load for patched ball and 50 cal sabot 45 cal 240 grain.
Get you a decent load and can tweak from there.
Have read that some use 100- 120g in 50 cal with saboted 45 cal. Honestly, I do not get good grouping with that amount of powder. Going big bang not necessarily best but every gun different. I dropped the powder load and settled on 84g of "P" via my formula with 240g 45 cal sabot in 28" barrel of modern inline and it carries and groups better than the heavy loads. Found the same issue years ago with a Hawkins 45cal 1 in 60 twist, shooting 290g hollow base. Use 70g with patched ball and could hit 8 of 10 in bull at 100 yards. Eyes were better back then, LOL. But the 290g with same load would walk terrible. Dropped the load 20% and they dropped right in.
Anyway that's been my experience.