The fact that you were seeing a doughnut instead of concentric rings usually means you have either racked the focuser too far or you are not using high enough power.
The concentric doughnut is the least sensitive test for collimation. The concentric rings are more sensitive. Upping the magnification further results in an airy disc at focus which is very sensitive ( and very difficult to achieve unless you have a night of excellent seeing ).
Since your images are sharp and you are seeing good detail, I would presume your collimation is very close if not dead on.