Normally the bowl would not rock on a flat surface.
You can set the bowl over the flange without wax, just to see how it sets.
Then you will know if the wax seal is causing a problem.
Some wax with horns will prevent a bowl from dropping down far enough.
If the bowl rocks, because of ribs underneath, I haven't seen one that does, you could use shims at the back of the bowl, pinning the front of the bowl down in front.
Most of the time when there is a rocking issue, it's the floor.