Yeah it's a basic formula so cant really be wrong lol
Just the same as you filling up your tank with say 47L of fuel then driving 200 miles and running out, pulling out your calculator to see that you've averaged 19.1 mpg.
The computer of course does this live constantly monitoring your fuel flow and speed so can give you an instant figure as well as an average and predicted mileage left.

Would love to get mine working again!
Need a little convertor to convert injector PW to the correct signal the computer wants to see and of course be calibrated to match.