That's a little tough to answer too because you're going to make or have made the canvas top, so, to a degree you control the size of the opening. Do you already have your top bows? If not, I would suggest you get them, make them, whatever. Then set them on the car and use 2" masking tape to mock up the shape of the top. You accomplish several things. First you can determine how you want the top to look. Second, you'll figure out if you need to modify the bows to get the shape that looks best. You'll figure head room. And of course your window opening size and shape. Plus, with a DuVall, because of the shape at the top, outside corner, you're going to have to rig some sort of a, roughly, triangle shaped filler if you want a full seal, or make a "wrap around" header, which again will affect your glass shape. It all comes down to how you design your top first.