I prefer to make larger and cut down so the Quilt In A Day triangle square up ruler is a must have. It saves so much time and makes perfect HST.
Also like to make 8 at a time. For 2 1/2 in HST use two 6 in sqs. Draw an x and sew on each side of line then cut in half both ways then on lines. Add 2 in for each size larger (8 in for 3 1/2 )
The ruler has you cut once to square up. I did a demo at a meeting and everyone went and bought the ruler.