How to figure cost of pumping one gallon of water

We share a well. The primary guy pays the electric bill and we meter my renters water at that house by the gallon. Its been zero for years as I have another well on the rental land that recently started pumping nasty clay, and finally the pump inlet [I am guessing] is clogged.

I originally drilled the well and built his house, so I know its 220' deep and makes endless water of bottled quality. 1 HP pump, unknown standing water level.

Would several fills of a 5 gallon bucket with a stopwatch and ammeter clamped on the feed wires give enough numbers to juggle and then divide into the KW cost?

How would YOU do it?

At 10 cents per KW, a 1 HP will burn 10 cents per hour, $2.40 per 24 hour day. At 10 GPM that would be 17 cents per thousand gallons.

Bet that makes the guy in the city wish he had a well.