Usually...usually...the side cuts are on the same plane as the hinge. I almost always use a Humboldt face.
It depends on the tree...size, species, amount of lean, soundness of wood, but I usually make the back cut level with the others. I'm not saying that my method is carved in stone or is the absolute best...it's just what works for me.
The thing to remember on a heavy leaner is that when you start backing it up you're committed. Don't slow down or stop to admire your handiwork...hammer on the saw. But hammer carefully, don't cut through your hinge. This usually won't be a problem because the tree will start to move before you're really ready for it to.
Remember too that a leaner can still 'chair on you even with the best of methods. Stay focused. Also remember that a 'chair doesn't always slab out straight back. They can break out an an angle and come at you that way. They happen in an eye blink. A 'chair can also break at the top and go any way at all. Again, in an eye blink.
