The exact formula for the starting squares is the FINISHED desired size of the block PLUS 7/8" .
I prefer to add 1.25", make the HST's, and then trim the block down to the UNFINISHED size.
For example, if you need a 4.5" FINISHED HST block, you cut two squares that are 5.75", make the HST's, and then trim the 2 blocks down to 5" UNFINISHED.
@Kathy- you would trim down to 5" (the unfinished size).
Otherwise, it's the same concept. I just like the little extra to work with.