You've answered your own question.
Pipe #1-Unit#1 is the furthest
luminaire stage left in that location.
Pipe A-Unit#1 is the furthest
instrument downstage in that location.
If a
fixture falls on an intersection (which it won't as it MUST hang on one pipe or the other) pick either one.
If it's in the round and there is no "
downstage" or SR/SL, number and letter from the control booth's perspective (or the
tech table's vantage
point) at the bottom of the drawing.
Sample:
InstrumentSched.pdf .
.