You’re making it a little too complicated. There are multiple ways to get what you want. One that I’m suggesting is getting your location values from the elements (based on project coordinates) and then adding the survey coordinates translation.
Example:
If your beam is at 15’-0" from the PBP (0’-0") and your SP (1200’-0") is 100’-0" below the PBP, then your beam is at (1200 + 100 + 15 = ) 1315’-0". You just have to get the locations of your coordinate systems.