you can always take my approach which is to be thrilled if, when I measure top/middle/bottom to get the average so I can add the border, I come out with 1/2" difference or less

And then if it's more I just find the shortest block sticking out the edge and slide mat & ruler under/over and cut so that all the edges of the blocks match that one and thus my 1/4" seam for the borders will work.
But then, I am a terribly imprecise quilter so perhaps not the best advice to follow........my quilts are made to be used & loved, not hung in a gallery or show, so I completely don't mind the imperfections. I do realize this approach doesn't work for everyone, though