Since your options are so limited, being pellets and a conical bullet, leave everything alone and just aim 3-4 inches low on your 1st clean barrel shot.
Had you been using loose powder and a wad between the powder and bullet, you could fine-tune / tweak it a little. I suppose you could add a wad in shots 2 and 3. That may lift your target holes a little. But there's nothing you can do about the powder, unless you switch to multiple 30 grain pellets, or drop to a 30 and 50. Bullet weight/grain changes would be significant with Powerbelts. You are looking at give-or-take 50 grains moving up or down.
Not much you can do with your current setup otherwise. If it shoots tight groups, I recommend sticking with it and compensating your aim on the 1st clean barrel shot.
BTW.... if you don't own wads, just fill the Powerbelt bullet bottom cup with bore butter / wonderlube. That'll help cut the escaping gas.
