A double sided binding is a great addition to a quilt. It takes some fussing to get the seam to line up right on the edge of the quilt. The extra work is well worth it when you see the results.
My notes are to start with a 1" strip for the fabric you want against the top of the quilt, and then a 2" for against the quilt back, using a 1/4" seam. However, those measurements can change according to the thickness of the batting and fabrics, even the thread! Also, your seam allowances for the strips and when you are sewing on the binding can dictate a change.
It's best to make a prototype to check the widths of those strips. Be sure to use the same fabrics etc. as the quilt. Otherwise, it might sit differently on the quilt edge when you go to that step.
The difference to your calculations .... you calculated 1/2 the binding adding the seam for the seam. However, remember, 1/2 the binding gets hidden inside the binding. Thus, the 1" and 2" strips.
Hope this helps.
You can do it! and will be SO pleased with your results!
Go for it!