Or a more realistic case if you also have a bug or a database corruption.
I would exclude this first. The reliable metric would be only the stat after payout, when all data is settled and your node will receive a receipt in the first two weeks of the next month.
You need to take a snapshot of the bandwidth usage on the dashboard (both graphs and also a table) in the last day of the current month, then compare it with the usage for the current month in the next month, when the payout will be completed.
You also need to take a snapshot from your router usage (but it can be useful only if there is no other internet consumers except nodes).
You may also use Earnings calculator (Detailed earnings info and health status of your node, including vetting progress and garbage collection status), but please note - the estimation will be replaced with a real usage, when your node will receive a receipt.