The only way you will know for sure is to batch test with the pistol secured to take out the human error, then batch test at least 10 pellets of different makes / sizes. pick the ones that give you the tightest group and stick to them otherwise you will always wonder if its you or the pellet.
try not to buy all the pellets for testing, ask other people if you can have 10 of their pellets to try.

HTH