Hours into a per-minute average

Hi all, sorry for the very dumb question from someone who just registered.

I have a very basic spreadsheet calculating the amount of water used for a job. It looks like this:

Start time (HH:MM)
Water meter - start
End time
Water meter - end
Total water
Total time

I am trying to find a formula to add a cell showing "Average water rate" ie total water / total time. Though my start and end times are HH:MM, I would like to calculate my average water rate in units per minute.

Sorry if this isn't very clear!

Thanks!

I tried that, but my answer is then either in a time format, or if I format my answer as general, it is a completely wrong number.