Folded corner tips ?
 

I'm doing a folded corner on rectangle strips. Maybe one out three will line up on the fabric edges to make a straight rectangle. I'm using the Folder Corner ruler and I like it, save a lot of time of not drawing a line. I've tried different ways and never can get the rectangle even.

https://www.fatquartershop.com/simpl...ners-ruler-aqd
maybe this will help get you more info. i don't have that ruler myself.

I've found that you have to go scant with this ruler. I don't aim for the point, but to be just outside the point at both ends.

Make the corner fabric a bit oversize and trim down to size.

Okay, I found my perfect placement. Thanks Joe's Mom. I put the top corner about 1/16 inch from the top and it sews perfectly every time. It only took about a dozen tries to figure the placement out. LOL