Joe, the loop flap as shown in my photo is first sewn grain side to grain side with the top line of the loop strap skived to zero so it will lay nice and flat, along a line slightly above and parallel to the angle stitch line seen later in construction. (Cannot be seen in the previous photo, but it's there...it seems like you are putting it on upside down). After that first stitch line the loop flap is pulled back over itself to expose top grain to the outside and fitted using a 1 1/2 inch belt dummy and marked for cut off at the proper length. After it is measured and cut to fit, I skive the bottom of the loop strap off to zero to make for a nice looking edge to the sheath.
Then backing off 1/2 inch from the edge the triangle stitch line is started to run parallel with the sheath edge. This gives the whole loop terrific strength. The other two sides of the triangle stitching are dependent on the angle placement which is placed to guarantee at least 1 1/2 inch actual belt clearance in the loop. A picture is worth a 1000 words so when you look back at the photo it should become very clear.
This whole deal can be set up at any angle you choose from flat horizontal to 45 or so degrees, but once you make that first grain to grain stitch, you are committed. You will note that the last angle stitch and the top line of the loop are exactly parallel.
A couple of different photos below may make it more clear. First photo is the first step of the operation, second is after loop is fitted and stitched. Final edge stitching has not yet been done in the second photo.
Paul