Short answer, yes. Long answer, by exactly how much to reduce the charge is debatable. I've seen many say 10% but I believe it is more. The first edition of the Lyman BP manual shows 100 grains of 2f pushes a .445" ball from a 28" barrel at 2008 fps with a pressure of 15,100 CUP. from the same barrel just 75 grains of 3f gave 2025 fps with a pressure of 14,690 CUP.
Now the second edition of that book shows a .440" ball from the 28" barrel 100 grains of 2f hits only 1881 fps with a pressure of 18,800 PSI, while 80 grains of 3f goes 1861 fps with 15,300 PSI.
Stepping up to .54 caliber 100 grains of 2f gives 1592 fps while 80 grains of 3f goes 1590. So I'd say reduce your 2f charge by 20-25% when switching to 3f.
Notice that in the first edition pressure was measured mechanically and expressed as either "lead units of pressure" or "copper units of pressure", where as the second edition measured pressure with electronic transducers and expressed pressure in "pounds per square inch". The two can not be exactly correlated. Also, the first edition used Gerhart-Owen powder while the second edition used Goex.
I like having both of those books. The second edition has more loads with a variety of projectiles which weren't even available to the first edition, but the first has very useful charts of downrange ballistics going out to 300 yards.
