I hate to take exception with the Graybeard's generally fine advice but I believe .410 is the exception, I don't think I have ever seen a box of .410s marked with dram equivalents like other shotshells. I would suggest you make or find a volume measure to hold 1/2 ounce of shot and use that same volume of black powder, just as muzzleloaders are normally loaded, equal volumes of powder and shot.