I don't understand why the face cut is only 1/3 on a straight tree if you are not using wedges. If the tree is perfectly vertical, healthy and balanced it seems to me the tree would fall backwards and pinch the blade. Or is this only done when the tree is leaning in the direction of the face cut? What's up?
So...
There is no such thing as a perfect tree, there are many that are close, but every tree has some sort of favor to one side or another
the 1/3 rule is like piracy laws, just a guideline, sometimes you wan't less then a 1/3 sometimes closer to 40% or even damned near 1/2.
the assumption is that you will use a wedge, even if the tree is perfect, the weather isn't, the wedge prevents the tree from sitting back, even if its just palmed in, then once you cross centerline or even not quite centerline, gravity from the face cut takes over, pulling the tree to the desired direction, but without that wedge, it more then likely will set back
I'm guessing this is another one of those instances where some grandpa somewhere made some **** up and since grandpa was old, everyone believed him, and didn't bother to look for their own answers, the theory more then likely being reaffirmed by dumb luck or subconsciously falling timber with the lean, and never needing to fight the lean
Now having said all that, a relatively straight tree, with a deep face cut more then likely will go where you want it, in fact as a rule I tend to make faces way deeper then 1/3, especially on timber big enough to sink a wedge to the hilt, any extra weight I can get on the face side is less work to lift over.