Colin,
I think I might see what is going on here - we could both be right.
On my database, the first form is just a maximized form and the second is a Popup with a sizable border. The adjustments I need to make to the positions to get the popup to align exactly where I want it on the main form are -150 horizontal and -15 Vertical.
You are dealing with two popups which have both a top and bottom invisible border (if border not None). Perhaps the vertical invisible border is larger at the bottom than the top.
So, in getting the second form to appear exactly below the first, you have to take into account the size of the invisible border at the bottom of the first and the invisible border at the top of the second.
I don't have to take into account the bottom border of the first form as the popup is wholly within the boundary of the first main maximized form.
So, I'm suggesting the invisible border of at the top of popups is 15 TWIPS and probably around 135 or 105 TWIPS at the bottom.
It's a pity it's not documented exactly somewhere.