Before I fit it, is there any trick to calibrate these things
without the use of a pressure gauge?

Set it to around 0.3 bar. Mine had markings for this purpose. It will start
opening even when not all the TRVs are closed, but this is a good thing that
reduces noise through the radiators that are still open. The 0.3 bar left
over should be easily enough to push enough water through those rads.

Christian.