I don't have a NM200 but I do have a NM145 with similar speed issues.
I have played around with the VBScripts in Mach3 and found the one that sets spindle speed. As is it requests the speed the user wants and outputs it to the machine. I was able to make a minor change to bring my speeds more in line by inserting a line between the two that are there:
RPM = RPM * 0.8
My speeds are rough guesses since my tach stopped working 1/2 way thru testing and I haven't replaced it yet. Based on your numbers I suggest the following:
RPM = RPM * 100 / 270
RPM = RPM * 0.37