A friend showed this method to me recently at the SewFree Retreat in Toccoa, GA. She had played around with it and said that whatever your seam width is when sewing along the raw edge of the fabric with your walking foot---is the basis for making everything work out easily. If you multiply that width times 4 that will give you the width of your bottom sashing strip. The seam width times 6 will give you the width you need to cut for your top sashing strip (then it will be folded lengthwise).
For instance, her walking foot gave her a half inch seam allowance. Hence, her backing sashing strips would be cut 4 x .5 (2" strips)
The top sashing strips would be cut 6 x .5 (3" strips which would be 1.5" when folded and pressed.) Hope this isn't too confusing. I tried it and it makes everything very easy.