You measure mid way between the G4 and tip of beam regardless of the drop
That would give you an H5 measurement. Do you mean the G3?
What if the drop is right there? Would you measure around it? Like, if it were a 12" drop, would you measure the 12 inches down, 12 inches up, plus the circumference of the top of the beam? You'd have a 26" difference in circumference just because the drop was a couple inches off.