Okay I think I have a Formula that works.

The Hex byte is going to be a decimal number from 0 - 255

x * -25 /256 + 7.5 will equal a number ranging from -17.40 to 7.5

Decimal 77 = 1.503 Volts = -.019 INH2O

This formula is close to what AE came up with. ... Login to read more.