Been messing again - it all seems relative to the spindle pulley settings.

I set the min as whatever I got after setting a M03 S1 command - this was 1500rpm.

The max was then tried setting above the actual limit - this made things worse.

Then I went below the limit - 20,000 actually did a good job at the 5,000 to 15,000 range and was pretty close BUT it means you cannot command full speed as you have limited it to 20,000 so not good Sad

I am leaving this at present as I am unsure it actually be done without a lot more knowledge and maybe some VFD tweaks too.