if you cut your binding 2 1/2 then it needs more than a 1/4" seam allowance to fit perfectly. 2 1/4 is better. However, test to see what width and seam allowance will give you the look and size you need. When I do 2 1/2" my seams are usually abut 3/8".
Best way to test is to measure your folded binding and divide by 1/3. This will give you a more accurate seam allowance. If you have to abide by a 1/4" seam because of piecing on tip then you should cut your binding a little less. However, I have seen many bindings that were wider on the back than front. However, since you say "the seam appears good but back isn't" that makes me wonder if the seam allowance is realy consistent.