Congrats on finding a good load that suits you.
If it were me, however, I'd try tilting the muzzle down, and then raising it to fire. Then I'd repeat with the muzzle pointed up, then lower to shoot. In other words, does powder position affects results. With that much air in the case, I'd wonder if it could be a problem.
With nearly fifty grains of long stick 4064 in a case that size I doubt there is enough air space to present any real problem.
