AppSheet: Monthly Water Meter Readings

I've created an AppSheet database to record monthly water meter readings. It has the following tables and fields:

User: Name, MeterID, Branch, Date installed
Readings: MeterID, Current Reading, Previous Reading, Consumption, Date, Days from previous reading

At the end of each month, I grab the monthly readings and export to the billing software.

However, I forgot something: Some of the meter readings are blank, and not included in the monthly report. This is because we can't always read each meter due to lack of accessibility. I know I need to calculate the estimate/average for each of those, but I don't know how to do it. I know how to create the formula, but I don't know where in the process I need to do it? Do I set all of the meter readings to calculate the average from the beginning, or do I wait until I export the data and then calculate those that are blank at that time?

Any help on how to set this up would be appreciated.