Every pass gave additional feedback for further adjustments. First it was clearance, then positioning, then depth of cut. The -0.375 mm clearance works well. The tenons still protrude because the flattening cut is of course at half the included angle. I decided just to skip the flattening pass and trim the extra with the band saw or plane before sanding. The other consequence of the adjustments is that the usable section length is shorter than my original plan. That just means that the fountain diameter will be less than planned by about 100 mm.