circuit,
Altitude vs pressure is a highly non-linear function. Your best bet is to use a look-up table. Consult NACA report NACA-TR-1235 "Standard Atmosphere-Tables and data for Altitudes to 65,800 Feet". My copy (an ancient one) shows pressure in mb (millibars), kg/m^2, and mm Hg (millimeters of mercury). By definition, a Pascal is 1Kg/m^2 (One Kilogram per square meter).
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The MPX4115A is calibrated in mv/KPascals (millivolts per thousand Pascals), so you can make the conversion directly from the table.
Regards,
Kral