I have a router using the G540, Mach3 and the SuperPID. Considering there are only 4 inputs, I did not see a need to feed the rpm into Mach. I calibrated the rpm of Mach to the display on the SPID using the pulley setting. My experience is that the actual rpm will be within 20 to 40 rpm of that set in Mach. I set the rpm in my CAM program (CAMBAM, by the way).

Here is a short vid that may help.
https://www.youtube.com/watch?v=x75SQLxLNv4

Rick