database modeling in php doubt

I'm going to make a system for a colleague so I'm in doubt as to how I model the database

I have a table with tbl_leituraText tbl_leituraPrevious tbl_consumo tbl_fatorCorrecao = tbl_total

How do I do this total get the result of this calculation and insert into the database ??? I'll do it in php

There is a picture of the system that I'm going to do in excel, which model I could use to make this system. When I enter the value, I want it to fill in the total value field and then have a submit button and save the data in the database.