I don't even think you Need to mark the sides of the Screen all that much, just mark the part of the top of the Cage (about 15-20 feet from home) where the ideal lauch angle ends and use the top of the Screen as the bottom range.
if you hit the top before that it is too steep and behind that you are fine.
ideally of course you mark the sides and the back/top.
If you don't want to mark it you could maybe say hit the back half of the top of the Cage (backstop to about middle of the Cage is front half and middle of the Cage to the back end behind the Screen is the back half)instead of hit the back of the Cage.
I made this Picture: the Cage is a Little shorter and higher than normal, so the Launch angle of the front part of the line is more like 40 degrees but you get the Picture.
