Copycat, most of us have been taught to sew the body of the top together, measure the two outside edges and also through the middle. Then cut the border length using the average of those measurements. You can still do that but you don't have to work with the whole top, just a few rows at a time.
If my blocks have some bias edges, which are prone to stretching, instead of just pinning at strategic places, I will mark the finished block size on my border. Example: if my block finishes at 12 inches, I will mark the border at 12 1/4, 12, 12 and so on until the last block at 12 1/4.
I also always sew a line of stay stitching about 1/8th inch from the outer edge of my borders--before they are attached to the top. I do not like handling the whole top any more than absolutely necessary.