In the past I've used max and min functions to calculate a range so I've extended that idea, not sure the logic is correct but have a look (unfortunately saw your question after the application of Guiness  which doesn't help :S ). There are other math functions so maybe there is a much easier way to do this.

b=max(0,a-9)
c=min(0,b-12)

d=(abs(c)/abs(c)) - Possible flaw, dividing zero by zero will cause a math exception so depends on how Poser handles this, I'm assuming the guys at SM will decide dividing nothing by nothing still gives nothing.

d=1 in range and d=0 otherwise

e=7-(4*d)