Material size recall via formula

We make use of the following pretty frequently

CalcNup(width, height, 12, 18, .25) - which from what I understand means:
Calculate the number up, based on the finished width and height (provided by the user), that will be printing on a 12 x 18 sheet of paper, with .25" gutters / bleeds / spacing etc.....

Is there a way not to dictate the page size and have it pulled directly from whatever material a user selected? We enter that information in the materials, so I'm not sure if there is a way to recall that information in a formula?

So, for example:
CalcNup(width, height, MaterialSize, .25")

This would save us from having to make duplicate engines only providing (1) size of stock each (so like an 8.5 x 11 engine, and 11x17 engine, a 12x18 engine).

Or allow us to not have to ask users additional questions to narrow it down, which would decrease the points of failure as far as human error goes.