How to determine the proper drill angle when drilling into a board that is not flat?

[John Pond]

I have a large slab of spalted maple that I planed about 6 months ago, meant for a live edge bench with a spindle support backrest, and a work in progress ever since. The slab is about 21'' wide and close to 9 feet long at about 1.75'' thick. It has been drying for 2.5 years outside and the six months inside if not more. I thought it was good to go, but my basement is moist in the summer and since cooling off the board has twisted/popped up at one end and a twist at the other. I placed it on some shims to determine a 'level' top, and what spots balanced at what heights and tried find a happy compromise for all the different angles. What I got was leg lengths that varied within half an inch, but that half inch occurs over the 21'' width dimension. So you can imagine that placing a router on it and drilling a hole will not make a mortise for the tenon that is perpendicular to the ground.
I have been a little stumped over this one and thought I would put it out there. Thanks in advance.

[Jerry Miner]

Here's a thought, FWIW:

Flip your bench face-down onto a flat, level surface. Shim under the corners or wherever to achieve the compromise position of the top (now bottom) surface relative to the reference surface.

After laying out the leg locations, set up shims as needed to level the router base, then plunge.

If you want to splay the legs, you'll need to determine the splay angle relative to the reference surface (which will also be relative to level, so a small level and an angled block can help line up a drill.