I've made 5 or 6 of these blankets and they are super simple. I cut about and inch apart and about 5 inches in. I eyeball it. I also lay both pieces of fabric together on the floor and cut them at the same time, that way you are guaranteed the same number of ties.
Oh, the other tip I have is to cut the selvage ends shorter than the other ends. The selvage ends stretch and you will end up with longer ties on those two ends if you don't cut them shorter.