I have done this a few ways. I like medallion quilts!
The easy way is to make the border, measure it, than calculate the size of the 'filler' border you need to make it fit, then put the filler border in first.
Another way is to 'float' the blocks with surrounding sashing or just sashing between the blocks so that they come to the right length to match the border. That is trickier because you have to play with the filler sashing size between the blocks to get the math right, but it can work when the sashing in the color of the outer bit of the block and it can be tweaked so the length of border is correct. And it may require a clever corner block or a plain filler block.
The worst way in my opinion is to try to change the size of the blocks in the border so they add up. That can be too much tweaking.
A clever thing to do is in the Stars for a New Day by Sue Garman where she floats the outer border of stars. To me, that is lovely.
I guess the net net from my view, is WING IT!
