I probably should let your comment go (cut 3/4 of the way through) but what you suggest isn't a good idea. That tree can come over backwards onto your tractor and you. The undercut should be on the side you want the tree to fall, and not 3/4 of the way through the tree (that is what I understood you to say). The undercut shouldn't look like the one shown in this thread either. It should be a 1/4 of the way through (somewhat depending on size of the tree and natural lean) and come to a precision point, and the backcut coming in about 1" above the point of the undercut. A hinge of wood is left, that can be used to control the direction of the fall. A wedge should be placed in the backcut so the tree cannot set back down on the backcut (as this will pinch the saw and worse, it will mean the tree has settled back in the direction you don't want it to fall. That is where and when people get hurt and get into predicaments they don't want to be in. There is a lot of weight to a tree standing balanced on a stump, and when some of that wood is removed, the tree is going to become unbalanced. Every move at that point is very critical, and once the tree moves in any direction (right or wrong direction), it will likely continue in that direction.
Please excuse the intrusion, but there may be readers that should know what the dangers are, and there may be readers that don't believe a word I say. So be it, but I'm hoping to keep someone healthy.
Falling trees is a dangerous activity. Knowing what might happen when 100's of pounds comes crashing down is the only smart way to go about doing it right.
Sorry, as I don't mean to lecture but I can't let it go unmentioned. Too many people think just getting hold of the right equipment makes them a safe tree cutter. The knowledge needed is vast, and without it, .... enough said.