Normally if you go in 2.5" you go 2.5" wide as well. Well maybe not normal, but if this is your first one I would sure recommend it. This way is for one that has a seam on the bottom as well as the sides. If you are making the entire front back bottom from the same piece of material you are correct. 2.5 x 5. That would give you a 5" wide bottom and gusset. Note that those calculations don't have any seam allowance figured in. Lay it out on paper first, then prototype it on something cheap, make corrections, do it again, when right, then you can cut the leather. Good luck.