Please think a bit about what the Clarity measurement is. It has nothing to do with subjective clarity. I think it is the most useless measurement, and I can't think of any application for it.
How REW calculates clarity C50: the first 50ms of the impulse is expressed as a % of total energy measured.
This is the problem: a 20Hz sound wave has a period of 50ms. This means that C50 only captures one sine wave at 20Hz. This would be "very very very early". At 10kHz, the period is 0.1ms. So C50 captures 500 cycles of 10kHz. 500 cycles should be enough to be considered "late". Go look at your impulse response or ETC and see how many reflections you can spot below 50ms. IOW: 50ms is "early" for low frequencies, and "late" for high frequencies. This is probably why REW has a custom clarity time.
On top of this - we know that with most speakers and most systems, the sound power is tilted downwards (i.e. there is more sound power at low frequencies, and less at high frequencies). So the Clarity measurement is severely underestimating low frequency sound power because only the first 50ms is captured.
And here is yet another point: in most typical listening rooms, the noise floor rises at low frequencies because rooms act as a low-pass filter. This is precisely at the same frequency range where output from the loudspeaker drops. As a result, the signal-to-noise ratio is typically very poor in the bass region. Now think about how this would affect your "clarity" measurement if the sound energy of the bass is barely above the noise floor.
As a result, ALL Clarity measurements should be upward tilting - less "clarity" in the bass, more "clarity" in the treble.
I don't think anybody knows what ideal "clarity" should be. If you add more room treatment, your "clarity" measurement will improve since reflections are suppressed and there will be less "late" sound power. If you measure "clarity" in an anechoic chamber, your measurement would look excellent. But we also know that adding more room treatment will REDUCE speech clarity after a certain point. I have looked for papers to see if there is a clarity target, and I haven't been successful. Maybe John knows of a study.
So: ideal "clarity" would only be interpretable above a certain frequency (we don't know what that is) and should have upper and lower thresholds (I don't think we know what that is either). Either the thresholds should be frequency dependent, or the "Clarity" calculation should be applied on a logarithmic scale (longer window for low frequencies, less for high frequencies). And finally: proper interpretation of the "Clarity" measurement depends on capturing a proper measurement with an adequate SNR at all frequencies of interest.
Because I don't find this measurement helpful, I don't look at it at all.