I use 2.5" strips for my binding and use 3/8" seam allowance but sometimes I still
have trouble when I bring it to the back. I think it depends on the thickness of
the quilt. Different fabrics can create different thicknesses. Piano keys were the
worst for me. I have to remember to cut wider bindings for such borders.
Making a practice sandwich like MTS said is an excellent suggestion.
When I make a practice sandwich (with the same fabrics/batting that's in
the quilt) I get much better results. Just have to remember to do it and
not rush to get that binding done...that is my problem. LOL