The Y Offset was designed in, but I can't remember why now!
Ah, I think I know why now... because when the figure is standing on it's head, your offset leaves enough room to see the head below the zero line. Maybe make the preview window two head-diameters taller? That way the whole head will show when the sholders' Y-coord is 0 or 255.
Maybe have faint lines (just slightly darker than the grey background) at Y=0 and Y=255 to show where you can't drag points outside of... only the head would be able to stick past the line.
Only cosmetic tho... I'm sure you're busy working on other things.