I'm writing a soaring forecast program that needs to know surface pressure at a specified terrain/site elevation. What is the best way to get this from the RUC/MAPS output grids or soundings?

My thoughts are to somehow find a "D" factor in mb (departure +- from standard atmosphere for any give elevation). Would the PMSL fields be appropriate for this?

I'm using the FSL formatted soundings from Moninger's site. Each MAPS/RUC sounding has a lowest level (surface?) pressure and the site elevation in meters. When converting that elev to std atmos pressure, the difference, doesn't seem consistent with the PMSL pressure field. Perhaps that elev is not consistent with the way surface pressure is calculated?

Any ideas?