Dealing with this earlier this morning. Zip hoods, but same idea, same skew. Any garment without a solid front
(zip, buttons, etc) is going to tend to sag in the middle, and you just have to compensate for it. Easy on a horizontal
shirt board, a little tougher on a hoop master etc. You are correct with squaring/marking. We use hypo-allergenic
soap and marker on the hoops.