RE: How do you know how much powder to use
Quick answer: Anything over 80 grains of black powder equivalent is MORE than enough to kill any deer. Don't buy into the 150 grain magnum hype. My elk load is 90 to 95 grains of 777. The only thing 150 grain does for you is make you flinch more and wastes money.
That said, If I could use a scope and sabots and longer shots were the norm, I might go up to 110 gr of loose powder.
The hard part is finding the most accurate load. Every gun is different and you might have to try several diffent powder and bullet combinations to find your guns pet load.