This is my tried and true method for HST (half square triangles)
If you pattern calls for a 3 1/2" unfinished HST, I cut my squares 4".
On one square, draw a diagonal line then sew a scant quarter inch on each side, press then cut apart on the drawn line.
Press (not iron) open then using a square up ruler, trim to 3 1/2".
This way if there is any stretch at all to your seam or pressing line, it will be squared up to the perfect size each and every time.