OK, I'm starting fresh and laying it out first so that we're on the same page.
My display and eyedropper are 8-bit sRGB at PC levels [0,255]. My source is v210, which is effectively 10-bit Y'CbCr at TV levels [64,940] with some values falling outside of that range that will have to be folded into the valid range or clamped.
The reference I'm using is:
Calibrating the luma gives me spot-on RGB24(0,0,0)/Y'CbCr30(64,512,512) for black; RGB24(255,255,255)/Y'CbCr30(940,512,512) for white; and RGB24(191,191,191)/Y'CbCr(720,512,512) for 75% gray.
Unfortunately, this also brings my red, blue, and green down to well below 191, respectively.
I really don't know what to do at this point. :-(