Getting the right amount of dubbing for a body is just practice. There is a cheat though. Dub your thread, wind the body adding or taking off as needed. Once you are happy with the body undo it. Take a small piece of card. Put one corner be the top of the dubbing noodle. Mark the bottom of the noodle on the card. Each time you come to dub the body dub the marked length of thread. Soon you will not be using the card, don't throw it away. Write the fly name on it, the hook model and size, and also mark things like wing and tail size, thorax length, hackle size etc. on the card. Keep it, because in 6 months, when you come to tie the fly again, you will have forgotten the sizes.
Here's a mock up of one of mine that I use for Wally Winged May Flies I tie every year for a shop.
Fly Gauge.jpg

The dubbing technique he used I have come across in a couple of books. It works with some dubbings better than others. It isn't the easiest thing to teach as it comes with practice. Far easier to teach people one direction first.

...but to save time in production tying. No reaching for scissors and cutting the tag or snapping to break off.
To make up for the time spent reaching for the whip finisher?

Cheers
A.