Since the stock gauge is not measuring pressure the reading if fine. The sending unit is a pressure sensitive on/off switch.
It can very safely be said that the stock gauge has a great bit of hysteresis built into it. Some have said it will zero out at 6 PSIG and otherwise read 'in the middle' but I've not read Ford documentation stating this.
As Paul indicated and Toyman inferred, don't read anything into it until you know actual pressure. You can have too much pressure, but to prevent this our stock bypass is set at about 60 PSI as I recall.