So, my research led me to this really useful document from the ITU - http://www.itu.int/dms_pub/itu-r/opb...2017-PDF-E.pdf
This chart in particular:
Now, for HLG, Reference White is at 75%. So, looking at my TV's performance, at 75% my TV delivers 208 nits. So, according to this chart, my TV is setup to assume a peak luminance of 1000 nits, even though it actually only produces a maximum of 820 nits.
Also, when setting HCFR to "override targets" for HLG, you need to enter both the Target MaxL, and the HLG System Gamma. This table from the same document defines what gamma to use for each maximum nits:
So for my TV, I needed to enter a MaxL of 1000, and a Gamma of 1.20. This gave a close match to the TV's actual performance, with a drop-off above 75% because of the limitations of the TV's 820 nits maximum luminance with a 10% window.
EDIT: If the TV has an Ambient Light sensor and it's enabled for HLG, then you also need to adjust the gamma in "override targets" to match the ambient lighting. In my case, that reduces gamma from 1.20 to 1.18 for calibration in my dimly lit nightime room.