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With small variations based on grain size, black power burns at a fixed rate.  What needs to be determined is the amount of powder that will burn in the barrel of your rifle without any leftover to burn beyond the muzzle.  The variable affecting the calculation is the caliber of the bore.  So, for a .45 caliber barrel, 32 inches long, 70 grains was determined to be the "no waste" amount of black powder.  There is also a slight difference in the amount of powder to be used to accomodate heavy loads as there is a bit greater initial backpressure (time delay) to move the load down the barrel.  Once you have determined the no waste powder load for your rifle, you can slowly back the powder down to determine what is best for your rifle and load.