The mirror happens when someone's using lunar years instead of solar, as you are doing. So using lunar, the 'new' year begins 5 days too early. This is why Christ died on True Passover but celebrated official Passover 4 days early, as I tried to show in http://www.brainout.net/PassPlot.htm
. The Jews had not intercalated for solar. Current scholarship seems to conclude that they only intercalated to create Adar Sheni every three or six years. The Bible shows that only one year wasn't intercalated, so apparently it's the first of a new delay cycle by the Sanhedrin. (Compare the wit John 12:12 compared to 13:1, skip of five days (4 in-between), then end John 18 versus Exodus 12, and you'll see he concatenates 4 days, playing on the 4 days from Arrest to Crucifixion as Preparation of the Lamb versus Slaying.)
So: if using lunar, there's a five-day 'drift' by year-end, so by three years, that drift is 15 days. 11+15=26, but as you know Jewish time a new day begins piggybacked on the old day, so then 25. Technically, 24th after sundown, so if there's a leap day or some other drift in the vernal equinox then 11+15 can equal 24 or 25.
Remember, the numbers would be there so the Trib folk can calculate where they are. So for all we know, this brainstorming thread will end up having at least some right answers they will somehow find. There have always been and will always be a lot of false ideas. So here, the calculation METHOD being fleshed out, will likely help. We won't be here. They will be alone. How they will find this thread when you and I will be dead, who knows.
If I recall correctly, at the 2009 Chafer Seminary conference or maybe in his studies at that point, Robbie Dean posited a hiatus between Rapture and Trib Begins, kinda like you are doing. I am not sure of the details.
EDIT: notice that a mirror can work in reverse, so now we have either a Chanukah start or a Pentecost start to Trib. I pick Pentecost as it makes Church time 'even', starting and ending on the Harvesting of the Gentiles. Now, that plays into your posit of delay (which I don't hold to, but Robbie Dean does), for then the time between Pentecost and Rosh HaShanah is likely for the "hiatus" (Dean's term for it, as I recall, but you can find out from his church, I don't go there).
So now, back to Noah. Gen 8:5 (use LXX, not BHS). So the gap is 93 days. It won't work, from Pentecost. By contrast, if 12 Chislev solar=Chanukah lunar is midpoint, then +93 takes you to 15 Adar, the 'sixth month' of the civil calendar, too, aka Purim.
Or something like this. For you really hit the nail on the head when you posited a comparison with Noah's timetable. ONLY GOD CAN ORCHESTRATE TIME. So no matter what excuses people come up with, if stuff that happens is otherwise explained away, then how come it's always happening on BIBLE dates? Hmmm? That, no one can invent or explain away.
Of course, our problem then will be that we've messed up our calendars. The Kaluach is way off since Josephus, 346 years too short, they cut out the Persian years (Google on that; so it's really Year 6121, not 5776). Pope Gregory cut 10 days from our calendar in the 500's (I want to say it was in 582), and I forget all the adjustments after that.
So God will be doing it likely at a poetic time for doctrinal meaning, but many who don't care to learn the BIBLE's calendar will miss it. In retrospect -- meaning, for those then alive and learning Bible -- it will make perfect sense.
This sounds like predicting the day or hour, but in fact you see we cannot: if only because, we've messed up our calendars so much.