I have made patterns where a half square triangle was used on a larger piece and the method used was to put the smaller square on one corner of the larger piece, (sometimes the larger piece was a square and sometimes a rectangle). In any case the outcome is a triangle. I found that to get the corner of the top piece to match the corner of the bottom piece after it was pressed into a triangle shape, I needed to stitch to the right (or closer to the corner side) of the line that is drawn to get them to compensate for what you lose in folding and pressing and come out more even. It also helps to use a finer thread like Aurifil 50 weight (LOVE that thread) so less is taken up in the sewing process.