Model builder: how to execute a calculation using two derived text files?

Hi there,

I'm working in model builder. I start with a raster input and after a bunch of tasks my outputs are two text file (two Surface Volume result text files). The actual final product I'm trying to get to is a ratio (calculation) between two values, one from each text file (the 3D_Area values).

Does anybody have any tips of how to set up this last step?

Thank-you!
Cheri