EDIT NOTE: I added a comment to the end of this from a later comment because I thought it should have been included in the first place. In this example 0-1023 is mapped to 0-255. The equation gets it ...

As C1 and C2 are to be determined, Arduino has the map () function that calculates the two variables runtime from two given mapping points (I1, O1) and (I2, O2).