For general use in the field and shooting at clay targets, BP shotgun loads can be done easily by using equal VOLUME of BP and shot.
Using the same volume measure you use for the shot,,what ever you choose to load,,say 7/8 oz scoop measure.
Use that same scoop measure to load that volume of BP in that load.
You don't have to be concerned with how much it weighs (unless that sort of thing bothers you). Just use that same volume of BP,,then the correct wads, then the same volume of shot.
Yes you can increase the load as needed in small increments to adjust to your needs (patterning, power, range, ect). But for a starter load they work just fine and will break clay targets on a skeet field or out of a hand trap all day long for you.
Keeps it simple,,the way the sport is supposed to be.