Ian,
I would suggest using the OEM "Encoder" DROs to start your value, then you can use the button functions that Zero these same encouder DROs, which in turn would zero your User DRO.
further, I would recommend using the Plugin Serial modbus as opposed to the old style modbus, with the plugin you can bring in each of the Modbus types from the M11. I.e. cfg1 = 1000 outputs, dfc2 = 2000 inputs, 5000 = MPGs/encoders.
scott