Chris:

Your drawing to the right is correct as stated by Petrie in his synopsis of coincidence vs intentional.

Your drawing on the right would set a level at 154 Rc above the base...the top of course #104.
According to Petrie...the two shafts protrude at the top of course #102 on the north side and #103 on the south.

Error in measure is...shafts 55+ inches too low on the north side and 26+ inches too low on the south side.

Close Chris...very close, but there's another reason why they protrude at the #103 course level...and it requires a little "natural" science...not simple math.

Best.
Clive