I am trying to figure out how to change from multiplication to division gradually as $x$ increases:
Given: $$250 \le x \le 1000$$ $$480 \ge y \ge 120$$
What is the formula for converting $x$ to $y$?
Is there an easy way to do it with modular arithmetic?
If you want a linear conversion, you want the two-point form for the line. You have two points $(250,480)$ and $(1000,120)$, so $y-480=\frac{1000-250}{120-480}(x-250)$