measure from the outside edge of the under-tuck in the back to the outside edge of the under-tuck in the front. make that drawing on paper for reference. if you have graph paper, that's best, but not necessary.
measure from the outside edge of the undertuck on one side to the outside edge of the undertuck on the other. again draw this on the same drawing, so that they overlay. measure exactly where the corner front posts are by measuring across the front from one post to the other. that will be where the slits (one side of the "T" opening) are. the slit is not really the shape of a slit. they are really the shape of an "L" that's been cut out of a square., or a dart, if you can picture that. picture a border, with the cornerstone missing. when it hangs straight down, it forms a boxy shape. that's what you want it to do. that shape is called a "T" quit because when you lay it flat, it assumes a "T" shape. you just have to remember a border with a missing cornerstone. that's the shape to make. is that easier to understand? if you can't envision it, i'll find a link.