How are you estimating 85%? Your goal should be to have the highest possible contact between the barrel breeches and the standing breech, while also having a high percentage of contact between the hook and hinge pin. In the real world 85% is good.
The only real way to determine this is to smoke it in with a kerosene lamp, a fat lighter'd pine splinter, or something else that makes a lot of smoke, safely. Smoke the barrel breech faces and the hook also to see if you're getting good contact in both places.
I've done this job before and failed to get the kind of results I wanted, so I know the pitfalls.
