App variable to be used in an equation

It would be nice if I could use a global variable in an equation in the data builder.

Use case:
We calculate for our sales price with a fixed exchange rate.
My solution would have been: make a table with one record only: the exchange rate.
Make a app variable from this record (this works already).
Alas in an equation I can not use this app variable.

Chem told me there is a workaround, but it would be great if you have global variables.

This is not the same, but also in the same direction: Page Variable suggestion

I’m interested in this too.

Chem did send me a suggestion around using JavaScript and the user logged in variables, but it was a bit too geeky for me. It made total sense the way he explained it, but it had a lot of moving parts and I was concerned about my ability to support / extend that solution.