Calculation between two non sequential rows

I have two wells that get a meter reading during the month. The readings will not be on consecutive days each week, nor on the same day of the week. Is an index formula the best to calculate the difference between the two readings? I then need to calculate the difference in the two reading to get the gallons per minute. I want to put a formula in there so that no matter which day of the of month the meter readings are taken, it will calculate the numbers. This is being used by field operators with no excel experience. trying to make it easy for them. thx.

I made some notes on my example. I need to calculate the time difference within the meter column. I then will calculate the gpm based on the hours converted to minutes. It's calculating the time in a formula that can run down the column. I may be overthinking. thx.

That is amazing! It works. Thank you so much.