From experience - Let the pump gauge settle for a second between strokes; mid-stroke readings can be off by about a PSI. If you don't have a reference gauge do a quick deflection check: span the board between two chairs a fixed distance apart and note how much it sags under you at your preferred stiffness, then match that sag next time so yeah... also account for temperature-going from a cool garage to hot sun can bump a 15 psi board by roughly 1-2 psi, and a dunk in cold water will drop it again.
