If you only use the convertor for the mill, probabally the best way would be to get rid of it and run a VFD with 1ph in - 3ph out, once you calibrate it the speed will be fairly constant and a known as it will be tied to the frequency selected, and can be displayed on the VFD. I got rid of the Vari-speed pulley on my Excello's and used direct timing belt drive with VFD control up to twice the rated rpm of the motor (120Hz).
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.