Creating an equation for sorting

I am trying to create an equation to use in a computer program. The aim is to find the value of Z, if the values of X and Y are known.

The Z value is going to be used to order some data, so the range does not matter.

I would like Z to increase as X and the result of the Y EQUATION increase.

The Y EQUATION needs to be able to be able to fit the following:
- When Y is between 30 and 40, the result of the Y EQUATION needs to be at its highest.
- When Y is at 1, the result of the Y EQUATION needs to be roughly a quarter of its highest possible value.
- When Y is at 200, the result of the Y EQUATION needs to be roughly a quarter of its highest possible value.
- No result of the Y EQUATION should be less than or equal to 0 when Y is greater than 0

Any help would be a godsend.

Im not entirely sure what you mean here? Perhaps an example would help?