I personally use a Patchwork Seam Guide on my Bernina (
http://www.bernina.com/en-US/Product...ork-seam-guide). It screws into the bed of my machine & keeps the fabric going in a straight line right under the presser foot. On my old Singer, the guide is built into the foot (
http://www.ebay.com/itm/like/2807759...466&rmvSB=true). That makes sure that I keep a consistent 1/4" seam all the way to the end.
If the issue is that the two pieces are separating for that last bit, you could try running a small bead of glue/glue stick within the intended seam to hold them together.
Edit: SewMagic has a good point. You could just mark the diagonal line of a square & then sew 1/4" on either side of the line. The larger piece might be easier to manage than the small HST's. Just add 7/8" to the finished size of your HST. For example, if your HST finishes at 2 1/2", you would cut a square that is 3 3/8". Here's a tutorial (scroll down to "Method 2"):
http://www.connectingthreads.com/tut...ngles__D4.html