16grs Lil Gun under a 280gr WFNPB by Montana Bullet Works gave me 1224fps from a 5.5" Super Blackhawk (12 shot avg) and was very accurate.
15grs of 2400 under same bullets gave me 1218fps and 16grs gave me 1244fps for some comparison. BTW in case it matters that was during a 75 degree day last May.
I don't have much data for the 280gr due to settling on the 260gr of the same design. Hope this helps.