The simplest approach that would probably give acceptable results is to take an initial pressure reading when your program starts, convert it to an altitude estimation to use as an offset, and subtract that offset from subsequent altitude calculations. For example, after getting the values you posted, your program could then subtract -310.25 ft from (or add 310.25 ft to) the calculated altitude from then on.
Alternatively, you could provide the
pressureToAltitudeFeet() function with an accurate altimeter setting so that it can compensate for the actual regional pressure and give a more accurate absolute altitude; for more information, read the description of
pressureToAltitudeMeters() in the library reference. Then you could subtract your actual ground level altitude from the calculated altitude to get the height above the ground. However, I would expect the reliability of this method to be very dependent on how accurate the altimeter setting value is, and it would require you to always update your altimeter setting and ground level altitude for your location and time. If you only care about relative height above ground level, subtracting an offset like I described above is probably the best method.