What if the most reasonable answer IS to read the palm of your hand? I'm not a person of color, but I believe human palms are universally Zone VI.
Same can't be said of the 18% gray card. It's not necessarily Zone V, and if you treat it like that, then you have discrepancies to frustrate you. People who calibrate their Zone System to camera tests with gray card as Zone V rate their EI differently than box speed. Certainly it will not agree with the incident meter at that point.
These frustrations can interfere with your desire to "accurately interpret light intensity". You'll ask yourself "Why don't I get the same results when incident reading compared to when I meter the gray card?" Haa, I think you did ask that.
I have a graycard from Sekonic that has several patches of gray. Trying to remember which one of those patches reads same as incident reading in same light has distracted me before. It's somewhere near the 12.7% patch. But remembering to pick the right patch - or using White paper and remembering how many stops that is from gray, is beyond my ability when taking pictures.
Placing my palm on Zone VI is not that hard. And on top of that... I find it pretty well matches my incident readings.
No Bill Black palms are Zone 111 or IV depending on the shade of the skin tone.