It is my opinion that the notch (scarf) apex is what dictates the direction that the tree will fall. This is why most fellers are so anal about the apex. It is probably why a dutchman is so dangerous. The reason for amending the back-cut is to compensate for lean or off balance of crown.
My son just spent two weeks at the IVMAA (Industrial Vegetation Management Association of Alberta) course. This is for Utility tree pruners, and is very comprehensive in all aspects of the trade. He informed me that there was a guy instructing in the merits of a double notch approach. My kid was really concerned with the method and asked my advise. I am withholding a decision until he can show me on paper or in person. The way he explained it lead me to believe that the direction that the tree would go, must be related to wishful thinking. He has issues with explaining things at times.
My concern with the box scarf is that the location of the back-cut could potentially cause the fibers to separate vertically, causing the tree to fall ANY direction. This scenario would be amplified with an asymmetric crown or a un-expected gust of wind. Trust me I KNOW about the wind! This also relates to the knowledge of what the relationship of apex/direction of fall. I thought I needed to compensate for a crosswind once and proved that the apex is what sends a tree to where it will land. Nothing and no one was hurt, except my pride, BUT it was a lesson I will NEVER forget.
On another note, the box cut would be great for using the site lines on the saw!