All fixed
Added a momentary switch to reset FRO
Changed the on/off switch to lock out the pot or enable the pot input.
This seems like a reasonable solution to me because if the on/off switch is activated then the FRO is not changing state - it retains the last override rate from the pot and it is rock solid steady. I was getting a very small 0.01% variation as the pot state was refreshed.
Then once the on/off switch is active the momentary switch can then reset the FRO to 100%.
cheers,
Rod