in order to set your valve within the ASME specified tolerances, you would need to, at a minimum, subtract the equipment uncertainty(accuracy) from the specified tolerance. In this case if the specified tolerance was +/-3%, and the gauge uncertainty was .5%, then the set pressure should be set...
Packdad,
It is "implied" that, in order to accurately set your relief valve within the specified tolerances identified in ASME Section VIII, you will need to take into account (allow for) the pressure gauge accuracy.