I agree on the crossover. To test this you need to get that 35zero nailed down to one hole in zero wind. Then, make a pellet hole in something ten yards away. Shoot your next pellet at that hole. Carry on for five more pellets.

If the pellets form a trend going to the right (opposed to your leftward trend at 55) then you've got crossover.

Problem is, the amount of deviation will be harder to spot at ten yards than at 55.