Gas price brake new construction: How to determine the previous year's period?

Does anyone of you know the detail of how the "previous year period" of the 80% cap is determined for new buildings where no figures from the "previous year period" are available?

This is the only statement I found:

Do you also have the impression that the gas and electricity price caps are being heavily exploited by the suppliers for windfall profits? On the exchanges, gas prices seem to have been falling for quite some time now, but we (Leipzig district, postal code 0457x) now have to pay higher gas prices starting 01.01.23:

Electricity consumption price: previously gross 27.78 --> from 01.01.23 gross 48.06
Gas consumption price: previously gross 12.65 --> from 01.01.23 gross 18.96
(The basic price remains the same with both suppliers. We also do not have a special contract, but are still in basic and substitute supply for the time being.)

It is really unfortunate for us that our forecast for electricity is so low; the hobbies sauna and beer brewing could not yet be taken into account by the supplier.