Congrats on finishing your top! I don't like to do much math, and as a long armer, 'friendly' (read: waving at you!) borders can be challenging! I found this method in Quiltmaker a few yrs ago and it has stood me in good stead. It is the same as measuring thru the center, but no math.
Fold your quilt in half and lay it on a flat surface. Lay your borders (cut both sides at once) along the fold in the middle. Smooth the quilt and borders, pinning as needed. Trim the borders on each end. The theory is that both borders are the same and will aid in keeping it sq and flat. If your ends are not even, you will have to ease or adjust on that seam. Better a bit of fullness inside the quilt than a large wavy border.
This is just ONE method, and just because it works for ME, does not mean it has to work for YOU. Try one method and if that does not suit, next quilt, try a different one.