How to calculate run time from hour meter reading

OK, I did not understand that you needed all the run times and not just the current one.  Let me think some more about this.

My initial thought is to calculate the 2nd most Recent Read Date in the summary relationship a summary and then calculate the corresponding Record ID#, and then look that up down to the Read Records and snapshot that field.

Then use the snapshot field for the self related relationship to the previous read.