.760.
Gives you room for .750 holed rings.
Never cracked a .840 forearm..
I bore the bottom 6" at .825..
I core handles at a little over 7/8.
BTW, it.doesn't hurt to have a long boring bar.
5/8 minimum and up to 7" LOC
What happens when you're down and it doesn't look right?
Wait, it can't not look right because all association between the two balls are referenced while stood up.
What if you want to shoot a combo? You can't use contact point because it doesn't exist?
I blew out a bubinga once at the end of that hole once. . Not pretty.
Now, I route a 2" hole at the exit and use a jig to cover and hold the piece centered to the rear chuck.
Helps in many ways.
It's a dead ball.
Spinning balls and balls brushing the side rail can be unjustly rattled.
If the opening angles are indeed closer to 143*, it's off-specs.