If you only need to adjust +/- 1%, you can do so by modifying a value in the device handler on this line:
def pct = Integer.parseInt(value.trim(), 16) / 100
Change the 100 to either 99 or 101. 99 = +1% RH offset; 101 = -1%. Because of rounding and other math operations going on that I don’t fully understand, it’s not accurate for greater or smaller values of offset.