RRsBrat
Hourglass/Quarter Square units can be awful... they are one of the times I do my cut/sew large and trim down technique, which doesn't help the ones you've already made. So, I haven't actually tried it but by my first look at what I think you want done, I think I'd be coming up a bit scant too at 7.25. I'd start with a square at least 7.5, maybe even 7.75. With the idea that you are making four 6.5" units, the basic math for a 1/4" seam and how much to add for a triangle is 7/8ths, which you round up to 1" which makes for some (but not enough for me) extra. Keep in mind, that there are two sides to a seam so any extra we sew or cut is multiplied even if it seems tiny to us.
Glad your tape worked! On my old vintage machine many many years ago, I used a child's fancy bandage to mark my seam allowance. The bump of the bandage pad (which was still a couple inches from the foot/markings on the plate) was enough to keep the fabric in line. I just grab what for me is a handy bit of graph paper to get that exact width/distance.
As for me, I did get the quilting I got done yesterday undone yesterday, but that's as far as I got. Am now fortified with coffee and about to enter the sewing room again to put in the correct bobbin and re-adjust my thread tension and begin again.
edit: Forgot to add this link, I thought it was a pretty good discussion on the QST:
https://www.bonjourquilts.com/quarter-square-triangles/