Thanks for all the input – I decided to go with gallons (or liters) per hour. This was purely for ease of development – because I was already handling inches (or mm) per hour it was very easy to add a new ‘per hour’ metric.
I do have the ability to switch between English and metric, and from what I can see it works fine. What’s interesting is the algorithm doesn’t care if everything is in inches or everything is in mm – just so long as its always consistent. You don’t get to switch between them 😛