Hi LucyInTheSky, I got a different result by chance. I wasn't reading carefully, and accidentally cut them diagnolly (instead of cutting across like you did), I ended up with 8 "hour glass" blocks; using the same measurements you gave. For 4.5 inch squares (before piecing), I started with 10 1/4 inches, sewed diagnolly & then made 4 cuts, I think.
So, I am happy for the simple method & the formula you gave.
Thank you so much.