Currently in the Tube and Pipe Environment, locating dimensions go from the connection point at the end of the pipe (and not face-of-flange) to the connection point of the fitting or piece of equipment that is being located. See below:
This is impractical in many situations, specifically when valves and equipment are to be dimensioned from the face of the flange. What I suggest is an option to dimension as below:
This way, you won't need to determine the distance from the face of the flange, then subtract the face-to-face thickness of said flange to get your dimension.