Spindle too fast, wrong voltage going to VFD

I just finished building my CNC and now have the spindle up and running, but I have a problem. I'm controlling the spindle with PWM via Mach3 > USB Smooth Stepper > G540 > VFD. The problem is that the voltage going from the G540 to the VFD is too high, so it is causing the speed of the spindle to be off proportionally. (See graph). The VFD provides 12V (actually 11.85V) to the G540. The G540 then takes the voltage from the VFD and then reduces it proportionally. For example, I provide 11.85V and want 50% spindle speed, the G540 then provides 50% of 11.85V back to the VFD.

The problem is that the VFD is expecting 10V as the the max voltage going back into it. Thus, the speed will not start to reduce until I am about 86% of the max speed (when it starts to go below 10V).

Could I put a resistor or potentiometer in line with the 12V coming from the VFD to the G540 to reduce the voltage to 10V so the proportions are correct? How would I choose the resistor? What about providing an external stable power supply of 10V to the G540 input? Is there a way to fix this in the software?

Thank you for the help!

Update: I tried the 5V out, 5V in line on the VFD, but the G540 is always a little low, so the speed was too low.