This is the calculation I use, ties in with Scottish Power's and also the consumption of a 33kWh boiler. Factors vary such as calorific value/adjustment but it's a decent enough approximation.

- The Rough Guide
Meanwhile, with the additional £67 or so going into the bank account, thought it might be an idea to shove that away for the purpose it is intended by increasing the direct debit by that amount for the period it applies to.
Guess what in SP's on-line "manage your direct debit", we slotted in the £67 addition only to be met with Scottish Power's ever so friendly message, "to increase your direct debit (
to just over £200/month) we require an immediate payment today of £1500.
What's with that

Thinking about it they probably anticipate the ending of the support, and for the period after March they want plenty in hand for the 12 months December-end November 2023.
So instead of shoving up to £200 the direct debit for six months, which is obviously "an offer you can't accept", (when to do that they want £1500 today), the £67 assistance will just go in as a manual "overpayment" each month. Currently, still at this point we are £200 plus in credit.
REgards Neil