Ok, I will give this a go.
Firstly I would sew a tad wider seam so the blocks end up as 10 1/2" squares - finishing at 10" in the quilt, which is an easier measurement.
Your sashing could 'finish' at 2" - (making five 2 1/2" HSTs) or you could make two 5 1/2" pinwheels (5" fin) - per each side of the blocks, plus extras for the cornerstones. Don't know if you could really call that sashing , but I wouldn't want to be making 2 1/2" pinwheels though.
I guess it depends on how busy you want your quilt to look, but I hope I have helped.