Assuming sound wood, there's a couple of ways to look at skinnning that cat, Greg. If you go with the standard back cut, just dog in and cut up towards the hinge until you have room to set a couple or three wedges, bang 'em in and keeping 'em tight as you progress to complete the back cut, then alternate whacks until you stand it up...maybe 5 degrees back lean is doable. Maybe you could improve on that with hardheads, or better yet with mags...but not by much, maybe another degree at the very most.
You can deepen your face cut to something over 1/3rd of the tree diameter, shifting the center of balance forward a smidge...might get another degree out of it.
Now if you gut the hinge on that 30 inch tree, leave about 6 inches of hinge on each corner, otherwise do everything the same, you could up your chances another little bit...say another degree. So we're up into the 7-8 degree back lean range.
That's getting pretty intense, but not impossibly insane.
There is another way to progress the back cut on a back leaner that can give you another bit of advantage in standing up that tree with wedges. The key here is never let the tree shift back into it's lean even a tiny bit, which will happen when you back cut from the rear until you have room to set the wedges. If you keep the tree from doing this, you don't have to overcome that extra bit of additional back lean, which is the part of the wedging job that demands the most power.
Sooo...make your face up per normal, or a bit deeper, say 40% of diameter. Now gut the hinge, leaving 5-6 inches of hinge on each corner...do this from the front and only gut deep enough to get past your planned hinge thickness. Now bore in from the side aft of your hinge and set the hinge thickness all the way across. I do this with the belly of the bar facing the hinge.
Now, pull the bar and flip it over so the belly is facing out towards the rear of the tree. Cut out towards the rear keeping the bar pretty close to parallel to the hinge (dog in and sweep forward a inch or two, then pull the dogs and progress the powerhead aft an equal amount). Do this until you have about two bar's width of back cut.
Now, leave the saw in the kerf and set a wedge on each side of the back cut, between the bar and the hinge. Bang 'em in tight as a tick. Keep the load balanced between the two wedges.
Then progress the back cut again in the same manner, 'til there's room to set another pair of wedges, one each side, right beside the first two. Drive 'em in tight, re-drive the first pair, then re-drive the second pair...tight and balanced.
Do this another time, same operating plan. Your objective is to get the back cut in completely while never letting the tree set back even a tiny amount. Three wedges on each side should be enough for a 30 inch tree, because you want to finish the back cut with enough space between the tips of the last pair of wedges to drive a pair, or three even, into the back cut and directly towards the lay. I have been known to intentionally saw off the tips of that last pair once they were set to make enough room.
Drive wedges alternately, and always keep an eye out overhead when driving wedges...stuff gets vibrated loose and will fall on you. Drive wedges in an irregular rhythm, and let the vibrations die out after a few thumps.
All this, and you might get another 2 degrees, max. Bottom line, you would be very hard pressed indeed to stand up a tree that had anything over 10 degrees back lean with wedges alone.
That's my take on the subject...love to hear what others have to say, too.