I always use the old standard of as many grains of powder as the caliber of the gun. Fifty grains for a fifty caliber, and (in your case) 45 grains for a forty five. Remember the finer the powder, the more of it fits in the same space. So a charge of 3f will be a bit more powerful that a charge of 2f.
Maximum load is usually twice the standard load.
In pistols, halve both (25 grains for a Fifty cal, etc, with a max of 50 grains).
This is not a unvariable law, just a handy guide for starting loads.
Dave