Dave, what kind of a meter do you have on your ro system please?
I want to monitor my water usage and be able to seperate my buisiness from my domestic usage.
Ta 
Thinking outside the box, can't you use your own house water meter to calculate this?
For example, take a water meter reading, then turn on the external house tap that feeds your RO and let it run for an hour.
During that hour make sure no-one turns on any house taps, or washing maching/disher washer.
After the hour turn off the tap and take another meter reading.
Deduct one from the other and you know how much water you use for an hour. Using that 1hr reading you should be able to roughly calcuate your water usage for work only.
Just an idea.