I'm guessing this is for a smoothbore muzzleloader. If your match shooting you might want to weigh your charges...
Otherwise,
3/8 oz=25grs black powder measure
1/2 oz=35grs black powder measure
5/8 oz= 45grs black powder measure.
These aren't precise, but using a volume measurer you can't get 1/2 grain increments. Remember, black powder is no where near as sensitive as smokeless.
I know firsthand how confusing volume to weight measurements can be.
At a trap match I watched a shooter use an 8 bore. He used 140grs ffg and 2oz (by volume) of shot, using the same measurer
I was ready to argue that he would only get 8 shots from a pound of powder, because he was using 2 scoops of shot, by volume, and 2 scoops of powder, by volume
After i settled down and thought about it, it became clear he got 50 shots (approximately) from a pound.