There is NO overlap. The whole piece may be about 30" long or whatever you want, but the tails are only 6" or so.
Most people remove the whole cover. I pop the core out of the side of the cover at about 6" down the rope, do the splice, then bring the cover back up and stitch it down. You got to be right on with your measurements, but it gives a better finished product (tighter cover) so it's worth it for me.
love
nick