Yes I'm aware it doesn't help re zoom or APL, but it's some relevant info for the current models.
Re the politics, my understanding is that it's fine to repost the information as I did, not to link to it.
Re the difference between -10 and -15, it has nothing to do with the accuracy of the black measurement because both the T10 and the Discus are within their accuracy range and I don't have the same results depending on the calibration. Both the Discus and the T10 are able to measure reliably with the DI on (i.e. much lower black levels), so they really have no problem measuring black with the DI off in any calibration.
For some reason, in my SDR DCI-P3 calibration, closing the iris lower than -10 doesn't yield any on/off improvement (though I see the iris closing further and both white and black measurements go down).
I just did a quick test as I had my Discus out and it goes roughly like this: open 30,000:1, -5 40,000:1, -10 50,000:1 and -15 roughly the same. I measured -12, it's also around -51,000:1. Use correctly, the Discus is usually close to my Minolta T10, so at least they agree with each other.
It could be due to the use of the filter as it does increase native on/off. I did all my contrast and brightness measurements in SDR DCI-P3 (low lamp, filter) because that's the calibration I use for UHD Blurays tonemapped by madVR (hence the most relevant for me from a PQ point of view, as that's what I watch most by far).
I ran a few measurements in SDR Rec-709 (low lamp, no filter) and it gives more usual results, with a more significant different between -10 and -15:
Open 25,000:1, -5 33,000:1, -10 41,000:1 and -15 45,000:1.
As you can see, the calibration (especially filter / no filter, but also possibly the gamut used) has an impact on the measured native on/off.
Before you ask, black is set properly in both calibrations, so that's the optimal results in both picture modes.