Couple of different layouts for the HOUR GLASS block:
Is this your version?
http://www.redpepperquilts.com/2010/...-tutorial.html
It is basically a QST (Quarter Square Triangle) block set on point.
The exact formula is to start with 2 squares that are 1.25" larger than the size of your FINISHED block size.
I'd go with adding 1.5" and then trim down. Way easier.
So if you want to end up with 3.5" FINISHED blocks, then you need to start with two 5" squares and use the method in the above link.
Then trim down to 4" UNFINISHED.