You have set yourself a tough task. While there may be a tricky way do just what you want, the easiest thing is to have a separate parameter that allows the user to select a "default" or "custom" value. This would then instruct the code to calculate the value according to the zone number or allow/require the user to enter a value. The details depend on just what you want to do. There are too many possible variations for me to go any further.
