I've read that "many quilters" shy away from doing on-point settings because of the edge triangles. I haven't done one in many years but it is a very effective setting and there are many designs that wouldn't look as good if not set that way.
Since I am not using a pattern for this quilt I had to figure out what size to make the triangles. I haven't found where EQ6 tells you this info about a non-block area. I started by looking up what the diagonal measurement of a 8 1/2" block is, which is the unfinished size of the flower blocks. I found a chart here to look up the diagonal of a square and it comes up with 12". I tried that and it didn't give me the 1/4" seam allowance. I found that using a 13" diagonal came out a little bit larger than needed and that's what I wanted.
Another point is to never use a bias edge on the edge of the quilt, it will stretch unbelievably (I know this from experience). The popular way of making an edge triangle without having a bias as the long side of the triangle is to cut a LARGE square, cut that in half diagonally both ways, and you end up with 4 triangles where the long edge is on the straight grain. You can see that explained here and a way to calculate the size of the square to start out with.
Because I was only using one triangle of each color I couldn't cut 4 out of a square. I did the simplest thing I could think of; I measured and marked 13" along the lengthwise grain of a fat quarter, then used my 12 1/2" square and lined up the marks at the same measurement on two sides of the square, cutting out the triangle of fabric that it forms. Hokey... maybe. But it worked and I'm all for whatever works. There's no quilt police to come tell me I'm doing it wrong.
The corner triangles are cut on the bias since the bias edge will be sewn to a block and you want the two straight grain sides on the edges of the quilt. Mine are cut from 7" squares for this quilt.
It's coming along.