What Dunster said. She's right. IMO

Me, too. That's great information

This is what I do and it works like a charm. Since you will have some bias edges I starch my fabric first to give it some stability. You may need to trim a bit once they are set. Don't let the formula scare you. Just plug the numbers into a calculator and I round up to the next 1/4" to make it easy. So in other words if your answer comes out as 3.192 I would cut my square as 3.25 (3 1/4) and then trim once it is set.

I set many of my quilts on point. I use the Robert Kaufman Quilt Calc app on my phone to figure my setting and corner triangles. So easy to use. All you do is type the size of your finished block and hit calculate.

I think the corner triangles should be at least 33/4" personally I would do 4 inches. The quarter triangles I would use a 6" square cut into 4.
hope you manage.

When I trim a quilt that has setting triangles, I lay the ruler from point to point. Cut at least 1/4 inch from the point. A little more is better than less. If the setting triangles are a little larger, your quilt will appear to "float" and you won't loose even one point. One quilt I trimmed at 1/2 inch from the points. Worked beautifully.

Go to: www.quiltbug.com/Articles/on-point.htm. Ran a copy of the chart several years ago and would be lost without it. So much easier than remembering the formula and works every time. I adore on point quilts.