Manually adjusting energy helpers

The context of this question is I am monitoring energy usage on a monthly basis from a utility that has one tariff for < 500kWh of usage and one for above 500kWh. This utility also does not have a fixed billing period, it’s roughly a month +/- 1 or 2 days. So my sensors accumulating data are always off.

I have two helpers - below500 and above500 - that track the usage and one helper - totalkWh. The helpers have cron attributes and with an automation that switches from above to below, everything works fine. What I need to be able to do is adjust the sensor values of the prior and current periods once I receive the bill with the actual cut off date. Is there a way to do that?