BPS...If memory is correct the one used to kill Lincoln was about .40 caliber, I can't imagine Booth using more than 30-40grs...
I've also read that Lincoln also probably died as much from one of the doctors probing for the ball as the damage made...
To the OP, I'd start with 20grs FFF Goex and move to about 40grs...You can check penetration by shooting into gallon jugs of water or 2x4s...