A lot of people make their hst units a smidge larger and then trim them down after pressing.
A rough formula - If you want a 3 inch finished square, cut the squares 4 inches - then there is a smidge to trim off - a "smidge" to me is less than 1/8 inch
x = finished size
so cut
x + 1 inch for the squares to be cut diagonally
the usual formula is to add 7/8" to the finished size - when I do that I always end up with a skimpy square - so I prefer to cut a tad (a tad is about 1/8 of an inch) larger and trim down after sewing