When I sat my exams 2017-18, some subjects used 27 ft/hPa and others used 30.
Just so long as we all keep in mind that both are wrong except for the particular Hp values where they are correct (around 3000 ft).
For interest, I put some stuff which you might find relevant at the following link
Bob Tait's Aviation Theory School - DenAlt... 1 Degree C = 120 feet and also = 500 feet. WTF? - Bob Tait's Aviation Theory School Forums
which includes a link to another thread which has a graphic which may be useful
Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums
Some of the fanatical support for 30 ft/hPa I see around the traps beggars belief ...