Thank you for your post!
A great place to get references with descriptions and examples would be:
I am also going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive even more views and potentially more advice!
A simple method for your water meter readings is to set up a self join relationship that links a given record to the one immediately previous for the same metter. Then an auto-enter calculation can copy the previous meter reading into a number field of the new record to include as part of a stored, indexed calculation that uses the looked up previous reading with the new reading to calculate the desired result.