Surely a standard encoding altimeter set to the QNH would give the correct altitudes to the Transponder anyway.
Crab, as far as I understand, the raw ADS-B data is pressure altitude (referenced to 1013.25hPa) [in accordance with ICAO Annex 10 Vol IV.
3.1.1.7.12.2.4]. For an amateur receiver simply listening for the ADS-B data and feeding this into a website, there is no correction and to determine the actual altitude AMSL, a correction will need to be applied corresponding to the QNH that day.