I dug up some old BP loading data and here is what they give:
45-70-405 57 gr charge/405 gr bullet/1300 fps vel/1520 ft lbs energy/26.7 efficiency in ft lbs.
Efficiency is energy imparted to the bullet per grain of powder. In bp it is safe to use the energy figure which hardly varies so multiply the charge by the efficiency and you get the ft lbs energy at the barrel muzzle. Only on this cartridge.
Rock Lock