Don't worry about the math.  In fact, who cares about the math?  Forgetaboutit!  Make a paper template for a mock-up.  

Once you have that inner black border sewed on, put a strip of paper alongside.  Just draw the angle on the paper for the finished template.  Remember to add your seam allowance!!

Then, before you cut your actual fabric, do a mock-up or two in practice fabric to make sure that the angles really are correct.

I love it with the colors like that but it looks challenging!

for some reason I was thinking the lines weren't consistent, like radiating from the center.  But they are parallel.  not difficult.  I'm  almost done piecing the border, lots of bias, which is good and bad.  Gives me some wiggle room, but it does wiggle a lot.  I'm adding some stay stitching and lots of starch.  Now I need to decide just how wide I want the black spacer.

The scrappy stars top is done. Some of the lines in the border don't appear parallel in the photo but they are in real life.  I like how the border gives sort of an optical illusion of stripes underneath the black, extending out from the center.  And yes, I fixed the mistake where one block is turned 180 degrees.  My son gave me grief about the colors not being in the right order for a rainbow, so it's not really a rainbow quilt.  I am just going to do an edge to edge for the quilting, I want the piecing to be the focus.  I had a friend visit my sewing room for the first time recently.  She looked around at this on the design wall and the quilts on the walls and said "you sure have a thing for black and brights".   Yup, I do.

Absolutely stunning quilt, Dee.  I  love how you did the borders.  Who is the lucky recipient of this beauty?

That border is **stunning**.  The entire appearance is outstanding. Gosh, you do good work!

I think your border is perfect for that quilt!

LOVE LOVE LOVE!

Beautimas! Looks smashing...

thanks Lynne (and the others).  This one is staying here, unless deal daughter grabs it.  But she already has a closet full of quilts plus blankets.