Theoretcal formula:
Cut squares 7/8 " larger than you want the finished HSTunits to be.
Example: two 5-7/8 squares would yield two 5 inch Hst unis. ( 4.5 finished size)
this ALWAYS results in my HSTs being too small - even when I use super scant 1/4 inch sean allowances
If I cut my squares 1-1/8 larger, this gives me a bit of wiggle room.
Example: two 6-1/8 squares would probably give me two 5 - inch HST units if i use my " regular" quarter inch seam.
I think this something you might need to experiment with to find out what works for you. There are variations in rulers and cutting methods, seam allowances, and even pressing.
Unwashed fabric may shrink if it is pressed with steam.