@LondonSimmer looking at the files, I don't think the final prop heat amp value is actually saved into a convenient variable, but this is a simplified expression that is used to calculate value on the prop amp meter on the panel:
(A:PROP DEICE SWITCH:1, bool) 17.5 * (A:CIRCUIT ON:10, bool) * (A:PROP DEICE SWITCH:1, bool) 18.0 * (A:CIRCUIT ON:11, bool) * + (A:ELECTRICAL MAIN BUS VOLTAGE:1, Volts) 12 > *
The actual expression is much longer, and introduces effects like the needle wobble, but this should be just the plain numbers, if I got it right. Seems to work for me.