Being these machines do not have spindle encoders, it would probably be a good idea to use a RPM gauge with out a load, adjust the program, then run it on a scrap piece while tapping a hole and watch the RPM's, adjust again if needed.
I have a RF45, but they have simliar quirks of a Tormach. I find that my machines RPM will vary across the board. One of these days I should make a chart, mapping the input RPM vs actual RPM.