A simple linear interpolation where 0 is 0MPa and 1023 is 1MPa (or better 1000kPa).
So the formula is one for a straight line y = mx + b, where b = 0 as you have the 0,0 point as part of the line. m is the slope of the straight line or the proportionality constant.
Give this is the conversion, you should look at using the built in function map(). Look it up in the reference.