This is an option:
I found this is something I got better with as I kept doing it. I try to focus on physically going through the same motions when I tie the knot, as tying the knot itself is the biggest source of variability. I also usually tolerate up to a half inch of variance and always throw the shorter string on my non-dominant hand.
I got lazy and just bought Sochi Loop string. The length is maybe a touch longer than I like, but I can just string up and go.