a general guide is to cut your unfolded binding 6 times the width of your seam allowance and adjust from there. I had been cutting 2.5" for a 3/8" seam but found that a bit wide so now cut 2.1'4" and it is better fit.
When you get your binding sewed, then look at it and you should be able to divide it into thirds and take consideration in the space taken up in fold.