I am sure sanding with a shim on the heel block works great as you gain experience with it. I know Sylvan Wells does that too.
I actually do use a bar across the body as a pivot point. It is located to correspond with the front of the bridge and is roughly the same height as the bridge. The sanding block has a piece of 1/4" plexiglass attached to mimic the fretboard with 80G stuck to it.
Fine tuning is with the Fox paddle with the pivot point about bridge height but can vary based on your construction techniques and movement after settling in.
I use a Bourgeois type double tenon bolt on which moves a little more under tension compared to a glued extension and glued dovetail. By trial and error I have found that having the plane of the upper bout such that with no tension the fretboard plane of the extension would just hit below the top of the bridge works well for me. I overset the neck to give a good 1/16" clearance without frets and when strung up and settled in the neck set is usually good and the angle of the upper bout winds up giving about the right drop off of the extension.
I usually don't have to sand the fretboard or frets for drop off much these days but did before I figured out the optimal upper bout angle for my neck attachment and upper bout bracing system.
I think the optimal upper bout angle will vary with each builders construction techniques and depend on how much the guitar settles in over time. It's good to try to see as many of your guitars as possible back in the first year or two to see what they are doing once they leave home and modify construction techniques as needed. One thing none of us need is a rising extension one or two years out.
