"when the first sliver of moon was visible" -- by my method that would 1st day waxing
Right. Using the Nov/Dec calendars above, to my understanding:
The noumenia or the first sliver of waxing would be Nov. 24 or 25.
The 10th would be Dec. 3
The 20th or "earlier 10th" would be Dec. 13 or 14.
Then the final 10 waning days count down toward the next noumenia.
So the 20th day of the lunar month would always look the same.