Had you reached through with your bar at that angle, Gary, you would have had a 2/3 width hinge.
Cut the bark off the tree at the hinge, and the back where you will be backing up/ helping over your tree with wedges (which don't do squat in bark, except let the tree squat back). This will narrow the tree, also, as well as make it more clear what is happening with the wood (matching cuts, dutchmen) and see the exact hinge thickness.
If you're cutting a 21" tree, a 20" bar can be easiest to screw up with. Remember that the bar tip is curved, so if you're seeing the very tip, it will be easy to overcut you hinge. An inexperienced spotter on the far side can "help" you with this error in perception of the active hinge thickness.
Also, as a less advanced feller, mark your ideal hinge on the side of the tree. This will help you when you look around the tree, and for a spotter to help you. There have a concrete point as a target for the end of the back-cut kerf, unless you learn things while in the backcut that makes you change plan part way through (sometimes you're hinge is on the very edge of a hollow, but still conceals it. Look for discoloration in the hinge when cutting the face-cut).
I'd cut 6"-12 on the "offside", and then 16" from the 'inside', as Justin mentioned.Easy to keep the offside hinge square, and then I am sure to use my dogs to anchor and pivot as I approach my final hinge thickness, always checking my gunning sights.
Aim with your gunning sight as far away as possible, rather than close up. Your gunning sight will be aimed at two different points depending on which side of the tree you're aiming from. The will be a few feet apart (tree thickness plus distance from the tree to the gunning sight line, doubled). Does this make sense?
The corners are the most important part for steering a tree, which is why hollow or hinge-gutted/ Letterbox'ed for brits go to the lay. If you're corners are the right thickness, and it tapers a bit as you go inward on the hinge, you're probably ok. Avoid a Centerpost, a thick middle part of the hinge. Hard to wedge/ pull that over.