It would be helpful if you could state the whole voltage range of your pressure sensor, 0-?? vdc.
To maintain .25% accuracy there are several issues you may have address. Normally the Arduino uses the board's 5vdc voltage as the reference voltage for the A/D conversion and that voltage is most likely not 5.000vdc, but some value of 5v +/- 5%. Also the 5vdc board voltage will be slightly different if you power the board from the USB port Vs using the on-board 5vdc regulator. These things can be dealt with doing some fixed calibration adjustments in your software.
So again can you post the range of your pressure sensor is, as in 0psi = 0vdc to ??psig = ??vdc.
Lefty