Start with the units
Take the present meter reading, subtract the previous reading, and confirm the units shown on the bill actually match that difference. If the bill units are wrong, no calculator can rescue the final estimate.
Check the billing month
FPA and quarterly adjustments are time-sensitive. A calculator can only help if the bill month and the published adjustment month line up.
Confirm the status
Protected billing is not a decoration. If the last six months crossed the 200-unit line, the protected estimate is not the right one.
Look for the extras
Electricity duty, TV fee, arrears, and quarterly adjustments are where many quick estimates fall apart. Add them manually if they appear on your bill.