One factor on accuracy when using an absolute pressure sensor as an altimeter, is to be sure the sensor has internal temperature compensated or you are using external temperature compensation if not.
I found one easy method for a calibration reference was to get a portable radio that has AM aircraft band reception (116-136mhz). Any close airport, even many small ones have continuous broadcast of their local barometric pressure and if you know the elevation difference between that airport and your location you have all the information needed to set your starting pressure/elevation reference.
Lefty