OT, but I'd use a datastructure to represent your equation constants in a table, and then have an algorithm calculate things based on that table and the input parameters.
The if-comb is fine for simple things, but as the problem grows in complexity you're going to need to keep adding more if's. Treating the model as data instead of code will make things easier in the long run, but takes a little more work initially.
I'll leave building that datastructure up to you, but in my world an if-comb typically (but not always) represents a design that could be improved to be more algorithmic. A particularly noteable point is a variable such as '$ratioincome28', which probably should not exist as a named variable in your program. Rather, that value might exist as an array value at index 28 or (better?) a hash key/value pair.
In all, it's about maintainability and good coding practices for long term coding.