just my .02
i would not use fff in your rifle,,,,especially 90gr.....think maybe ff in a 54
you might work up from a smaller load,,,,starting down around 40-45 and work up with fff,,,,pressure with fff goes up quick
if you go straight to 80-90-100gr with fff......you might just see some over pressure and maybe damage,,,to you and the rifle
you know i love the 54,,,,,and my hunting load with round ball is 60gr ff goex,,,,it has never let me down,,,,and on a broad side shot i dont recall ever not having a pass through,,,for white tail anyway,,,,,if you are gona use it for elk,,,you may want to take it to the 80-90-100gr range
for me the 60gr is plenty,, and very accurate,,,,i would play with the load and find the SWEET SPOT and that is where i would load
you will be amazed at this cal.
my .02
ocharry