I usually cut my binding at 2 1/4" and sew the strips until I have enough for the entire circumference. (Of course, I fold in half and press) Generally, I sew it to the front, pull the finished (folded) edge to the back and sew it down by hand. Now I have managed (on occasion) to make the binding even but most of the time, the part that gets turned to the back is slightly wider than the part that is on the front.
I have followed the mathematical guidelines and they NEVER seem to work out. So the last few quilts, I did a bit of trial and error. I pinned the binding where I thought I would sew and pulled the remainder to the back. If it ligned up well, I used that line, if not - I adjusted the seam allowance.
I would suggest to play with a piece of binding and a scrap piece of quilt.
...and I just checked the link only to realize that you are talking about "self-binding" and I am very sorry but I don't know about those. The math should be OK. If your binding is 2.5" then you need to subtract the amount you need to turn under and divide the rest in half. You would need to leave that amount of batting all the way around the quilt.