Part Builder - How to incorporate invert elevation into model parameters?

[ltmullen]

I'm trying to make a custom structure to model a storm water treatement structure. The lower part of the structure will be relative to the invert elevation and will always be drawn the same way, while the upper part will vary in length depending on the design grade and the invert elevation. How do I base parameters off of where the invert is (such as telling it to change to a larger diameter X feet below the invert)? The answer isn't readily obvious to me after playing around with Part Builder, although I'd be suprised if what I'm trying to do isn't possible in Part Builder.

Thanks!