a good starting point for the average rifle cartridge is a powder charge weighing approximately 85% of the cartridge's water capacity
The water capacity can be obtained by weighing a case, filling it with water, seating a bullet, removing the bullet, wiping off excess water and weighing again.
Then look through the loading manuals and pick a powder that gives the highest velocity from a charge close to your measured water capacity without being a maximum load
. This is a starting point for a powder that is shown to burn efficiently in your cartridge.
You then adjust your load up or down
to find the most accurate load in your rifle.