[Ether]

Or in native units do this:

double P=200;
double T=1000;
double SetPoint;

void main(void){
SetPoint  = T - 1024*floor(0.5+(T-P)/1024);
printf("P=%f  T=%f   SetPoint=%f \n",P,T,SetPoint);
}

Output:

P=200.000000  T=1000.000000   SetPoint=-24.000000

If the sensor goes from 0 to 1023, then you have to go backward 224 steps (not 223) from 200 to get to 1000.

(Because 0 is equivalent to 1024, just as 0 is equivalent to 360 degrees).