So I always keep a constant IL even if diameter changes. Also so I can link my model dims to some text to indicate levels when detailing.
I thought of offsetting a plane from the cl and dimensioning to this but I'd have to ammend this after diameter changes.
Surely the 3D Sketch lines are what defines the centreline of the route. I need something on the invert. I thought there may be a way to select the invert in the model when adding a dimension.
As far as I know you cannot. How I have done it in the past is to create a plane that is the top of the pipe rack and then I just give my pipe route a distance above this of 1/2 OD.
Alternatively, you can create a plane in your flange/s that would be on the invert level, and then when you mate your 1st flange in place, the pipe centreline will be the appropriate distance above this.