When felling a full tree, avoid cutting the notch deeper than 33 percent of the diameter of the tree at the cut. Often it is not even necessary for the face notch to be made one-third, or 33 percent, deep to establish a hinge length that is 80 percent the diameter of the tree. On the other hand, some situations, such as notable side lean, may justify a longer hinge length.
That being said if you have a 10 inch diameter tree you make your notch 3.3 inchs your hinge will be just shy of 9 inchs or almost 90% of the diameter leaving you with an acceptable length of hinge wood. I had to draw a circle on paper and measure it' Like I said i didn't know the answer, untill someone can tell me a better reason for making the notch 1/3 the diameter. It could have something to do with the compression of wood fiber, for all I know
What I do know is that if his notch would have been 1/3 the diameter there would have been less wood to break and the tree might have followed his notch.