This has always been a problem invert labels for pipes in profile at he bottom of the profile.
These pipe invert labels for profiles are set for each scale plus 100 and 5 horizontal scale profiles. The user will have to erase the old labels if the scale changes as the placement is not dynamic. The label uses a hidden line placed at the bottom of the profile view. This is done with expressions. Credit for the math goes to Christopher Fugitt for getting me going down the correct path. I adapted his post on civil4d.com for labeling inverts at the manhole. http://www.civil4d.com/2007/12/expressions-and-x-y
Placement is pretty easy, make the profile and add the pipe network. Use cpj-prof-inv-in-50 and cpj-prof-inv-out-50 labels styles (or the other styles). The attached file has a sample in it. The labels will change if the invert or length of the pipe changes. This label also gives the invert at the inside edge of the structure.
I hope this helps someone and I did my math correctly!