Solved by markdaoust in post #6 
              
              
                So the “on” pixels have a range of 100-400, okay, I think I’m with you now. 

zero then normalize or 100 then normalize 

One advantage to using 0 instead of 100, is that it’s easy to tell that zero is not in that valid range. 
If you set the background pixels to 100, when you see a 100, it’s never…