Thats the way I have done it since i was a kid with a couple exceptions. When pulling up,with your foot on the tail, stop before pulling his shoulders through. While keeping your foot on his tail, pull the hide off the lower legs (his pants if you will) and then go ahead and pull the shoulders and front legs through and cut off the head. This will make it easier to remove the hide on the lower legs(pants) of tougher squirrels. All I have locally are fox squirrels and they certainly skin a little tougher than greys.
Also I think he used the outside of his foot to hold the tail down, I seem to have better control and pressure to keep the tail from slipping out by using the ball of my big toe (boot on of course.lol) to hold the tail down. Just my thoughts hope it helps.