Hi Mike,
We fly a baro as a backup for the GPS. We use a MS5611 because it is rated to 11 mbar, or around 100K'.  It's been quite a while, but if I remember right, there were 2-3 calculations based on temperature of the sensor to get from the raw value to mb.  Then there were another 2-3 calculations to get from mb to altitude based on the pressure.  In the end, we got pretty close.  This is a slide that I presented at the GPSL in Granbury TX.  This only goes up to 75K feet.  As it goes up, it gets progressively worse.  If I remember right, it is about 5K' difference at 100K'. Initially, we collected all the data from the sensor, then did the math on the ground. It took a few flights to get it tweaked in.  We don't correct for baro setting before the flight. That only seems to affect it by +-200'.  The goal was to get a good guess of the altitude as a backup so it didn't need to be spot-on.