Your original post question, leave the original pulleys reduction as it is now.
I have my milling machine set to the middle range. If I ran it off 60Hz 3 phase I think that setting was 1150RPM. I never have had the need to change the belt settings. I have a Hitachi VFD.
I don't think I've seen anywhere in this thread what brand and model of VFD you have. So getting help with the program parameters is not possible.
My VFD lets me display the spindle RPM, as it knows what Hz it is running the motor at, and there is a program parameter in mine that is a conversion ratio of motor shaft RPM, to spindle. It is very handy when wanting to set the SFM of a cutter like HSS to not exceed 100. I use the speed potentiometer on the VFD. It's another project to mount a remote start/stop, and speed pot. I just never get to it.