I use the same as you, 1/32 tapered ballnose.
The whole concept of lithos is to let as much light as possible pass through the lightest areas, that is why you want it as deep as you dare. Don't go deep enough and the light parts won't look very white, dark parts will pass no light at all. Cut too deep, well, then the project is ruined. You can use a belt sander to grind away material off the back if you don't go deep enough, but this can be tedious & messy if there is a good amount of material left. But I have saved otherwise useless projects doing that in the past.
I stopped looking at acrylic as a corian alternative a long time ago, suggest you do the same. Results are far and away much better and more predictable with corian. If anything, probably less stressful on the bit as well, and never any melting/gumming issues.
For that matter, "Corian" is simply a name brand of solid surface material, the original and most well known - Same as when we call any tissue we blow our noses on a "kleenex" when it can be any brand. [Perhaps you already know this]. Anyhow there are cheaper alternatives often more readily available, Wilsonart & Gibraltar 2 of the better known competitors.
This guy
http://candlestonegifts.com/candlest...r-lithophanes/ sells material that may be superior to all, and fairly cheap too. If you are serious about lithos, and for sure if you are going to sell them, the plexi will have to go. My opinion anyhow, I wish white plexi was a good alternative, my experience is it isn't.