If your looking for an accurate pressure reading, you need to take into account the temperature of the tyre (tire).
Ideally, you'd measure the pressure when the tyre is at its working temperature. ie: After you have ridden for sometime. Check the pressure again once the tyre has cooled, to get an ambient reading. This does give you a general base line value to start/check from before you ride. But, this would change (+/-) based on the ambient temperature.
I have a base ambient value, and a tyre warmer check value (rims are heated) this is constant and accurate. These values are usually tweaked based on track conditions and anticipated track temperature. Having a chat to the tyre tech/rep will give you an indication of what the track is doing).
Unless, you're using an inert gas, the pressure of the tyre will change depending on the temperature of the tyre.
Cheers