I once used Tartan’s gathering suggestion along with inch wide strips of a lightweight fusible interfacing cut to the length the outside edge of the triangle was supposed to be. I worked well however this was a strip quilt and not setting triangles and I didn’t have much excess to deal with. It might be worth a try.
If these are setting triangles I suggest for next time you google instructions for cutting so that the edges are on the straight grain. There are handy sizing charts that do the math for you.
I am not a regular starcher but agree if for some reason an outside edge must be on the bias, starch your fabric to death before cutting.