Hello - I want to make a lap quilt for my neighbour using this block

http://www.quilterscache.com/R/RibbonQuiltBlock.html
But - I would like to make the blocks 9" square instead of 12" so the pattern is more 'busy'; i want to speed piece the HSTs like the instructions say so what size do I need to do the squares for them? The middle square would be 3 and a half inches, right? Does this mean the squares for HSTs should be 3 7/8"?

I know there's a formula in one of my books for doing this but I can't find it! :oops:

I know there's some clever person here who'll have the answer in a flash! Thanks in advance guys! :D

K x