How can you scale 15 - 8 to 0 to one im trying this sort of math

You need to find the absolute difference between your two original values, as well as the absolute difference between your two target values. Divide these two results and multiply your input by that result, after you offset it to zero (in your case subtracting 15), and then offset it again according to your desired range (in your case 0).
Check out the [map] abstraction from here https://github.com/alexdrymonitis/miscellaneous_abstractions
That does it.

this has worked *-1 + 15 then / 7 is the difference