Modify a Pipe Label Style with an Expression
You could attack this Expression problem differently via the difference of between center to center distance and the inside distance and how that relates to the outside Pipe diameter.
I'd argue this is not an accuracy issue but perhaps a what is suitable issue. Obviously, the length of pipe sections used also bears on the construction and estimating problem.
In the real world you probably need the approximate overage. Whether the pipes are actually installed off the assumed center to center line is also an issue.