I had a look at this and only got a partial solution, but it may inspire you...
In the simple table I have made (Formula Headings and Expressions), the solution fails if you try and add the turnover formula to the table. To me it kind of makes sense that you can't dynamically alter an expression.
If it is just your intention to just have re-usable expressions, then you can achive this by just defining variables in the document and using them when needed. See the attached qvw for the examples.
Formulas.qvw 137.8 K
I think the only way to achieve this is a pick/match combination, like:
There has been a quite similar thread posted today, almost at the same time, so you might also check this.