Woody,
I see most at around 107mm above the mast step. The gooseneck is generally determined by the amount of spacing you want between the tack and the boom leaving sufficient room for the slight movement of the downhaul. The head should be almost touching and free of the crane.
The vang connection to the mast is usually at the base of the mast where movement is free of the step. You want as much distance possible between this and the gooseneck so that you will get the most movement up and down at the clew (tightening and loosening the leech)and less fore and aft movement of the gooseneck (bending the mast and affecting the shape of the foot).
Make sure the standoff line around the boom through the clew gromment is set to your liking before the position of the vang is determined. If that line in not in place or is later changed, a re-reference of vang will be needed.
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