If you truly cannot find another gauge you can still get close by doing a simple flex check at home. Set the board on the floor inflate toward 14 to 15 and stand at center with feet shoulder width and watch for sag the rails should stay straight and the deck should not potato chip under you. It is not precise but after you do it once at a known good pressure you will recognize the feel and the sound when you tap the deck. I still think borrowing a shock pump for one calibration is the best move then mark your gauge and forget about it.
