I am not much for gadgets.
But I swear by this one for accurate HSTs.
I love this stencil product:
Maker is Sunday Best Quiltworks
They make 8 or 10 sizes of this stencil which allows you to make 16 or 32 half square triangles at once that are absolutely perfect.
SO EASY AND FAST TO USE. You just draw using the stencil and then sew on lines, then cut on the other lines. Voila --- absolutely perfect half square triangles. Crisp, accurate, perfect.
This was such a relief for me I wanted to share it with others. No connection with company at all.
To find them, google Sunday Best Quiltworks. The company also has a youtube video that shows how to use the stencil, but it is extremely easy and pretty much mistake-proof.
best,
w.