Thanks folks! The top of the mortise is more critical for looks. On the bottom, it’s mostly a matter of having a flat enough surface for the shoulder of the cane shaft to mate to. And as I’m going to be drawboring the pieces together, I’ll have some pretty decent clamping force there.
The plan is to initially cut the tenon on the shaft to 7/16 x 13/16 inch, which is about 1/16 oversize each way, then nibble away at the tenon until it just fits. As such, I cut the mortise a hair wider at the bottom than the top (maybe 1/64 wider, according to my calipers), so I can do a very slight taper on the tenon as well, and get a rock-solid fit. My plan is to put pencil shading on the sides of the tenon, and slide the mortise and tenon together and see where the pencil gets rubbed off, and slowly adjust the tenon with floats. Should get me a pretty dang exact fit.
I can set my existing cane (from 2016) across two boards set 24” apart with the handle on one and the shaft of the cane on the other, and I can stand on the side of the cane. I figure that gives me plenty of strength for the finished product when used as a cane, even for someone quite a bit heavier than me.