You gauge isn't reliable. None of them are. That is why buying the calibrated 5-10-15 lb weight set for the Presto is so strongly recommended.

The weight, assuming we are talking about the 5-10-15 lb weight and not the standard 15 lb counter-weight that comes with the canner, is what is reliable.

So if you are using the actual Presto weight set and it is set on say 10 lbs. and the steam vent is open and working then when it is slowly but steadily rocking you are at exactly 10.5 lbs. pressure. If you gauge doesn't read that and reads more or less then your gauge is off by that much.

Now, if you are talking about the standard 15 lb counter weight instead then it doesn't rock until you hit 15.5 lbs. If you have had your gauge tested annually as recommended then you can go by the gauge. Run the pressure up to over 15 lbs and the counter-weight should start rocking. If it doesn't, something is seriously wrong so turn it off immediately.

Follow me?

Dave