I think Lyman makes a bullet mould for a slug that is used with a plastic wad, and their data should give you a starting place. I have loaded slugs of various kinds over the years, and I selected a powder charge based on the weight of the wad column and slug, usingchargesidentical to or slightly lighter than those I foundin manuals which were recommendedfor shot charge loads of the same weight. Never had a problem this way.

You could try doing this. But initially, you will end up with velocities lower than factroy ammo, since shot loads are slower than slug loads! In addition, you didn't give the gauge of the shotgun you want loads for, and the powder selection will be different for a 12 ga. as opposed to a 20 ga. A 20 will use a slower powder than the 12 does.