newbie skinner question, math module in xml?

So, i'm really trying to understand how things should work, i read and searched in the kodi wiki and didn't get an answer.
I want to set height for a control to be a CONSTANT\INFOLABEL, for ex. 10\10=1.

So, how is this done? it's such a basic thing to do in gen. coding but writing functions in xml feels like being handicapped.
I don't even know if infolabel if values are strings/ints if it even matter.

Should i use a built-in module?

thanks