You only need an external start/stop controls if you want a remote control of the VFD ie you want your cnc controller to start and stop the VFD. The pot will work without an external start/stop control.
Set Pd072 to what the highest speed of your spindle is or the highest speed you want it to run at is. Mine is set to 400 which is 400HZ and will be around 24000rpm.
PD073 is for the slowest speed setting of the spindle. Again set this to what you think the slowest speed you will use is. I think I left mine at the factory setting.