AutoCAD Inventor :: Piping - Isometric Dimensions To Elbows

I need a way to add isometric dimensions from the elbow below? 

This is how standard piping isometric drawings are done, no need for a front view, top view, etc.

Just all dimensions on one isometric view.

I am creating piping drawings for a Power Station and want to dimension from the intersection of the centrelines of all the elbows on the pipe run (just like dimension 300 and 1282 in the picture below).

I've been able to add dim 300 & 1282 by using "Include Route Centerlines" BUT if there's piping created without a piping route this cannot be done.

I don't understand why I cannot get dimension A & B from the standard work features displayed on the elbow or dimension between two elbows just using these work features?