I think 10% is the number. In other words, back off the volume by 10% when going to Fffg.
But the real answer is to bench the rifle and do a load development series of shots. Start at 45 grains of Fffg and shoot 3 shot groups and see what develops. Next group of three shots, increase the load by 5 grains. Repeat as necessary.
Time spent with your rifle is always time well used.
John