Thread: black powder data

So this is probably a daft question......how do you know correct charge amount of black powder in a cartridge if cannot find data for that????
specific reason.... loading for .45/70 AND YES I KNOW thats .45calibre and 70 grns black powder behind a .405grn projectile...but what if I change the projectile weight UPWARDS.......like to 486grn cast projectile..... do I need to reduce the charge weight??? strangely I cannot find smokeless data for that weight either....