If you have such values, then scaling will be slightly different.
for 0,25V = 0,11 bar & 4,75V = 3,06 bar please use:
1. a = 0,66 and b = -0,05 (with minus!) for absolute pressure
or 2. a = 0,66 and b = -1,05 for boost pressure when athmospheric pressure is 1.0 bar