I agree with Rich. Nobody knows for sure, including BCIS and GOD.
As far as I know/guess,
180 days count starts from Received Date. But it is always safe to count from Notice Date, which comes later.
When can use AC21 rule:
1. If you have filed I-485 with approved I-140 then 180 days starts from the Received Date.
2. If you file concurrently then
a) if your I-140 approved within 180 days then you can use AC21 rule on 181st Day.
b) if your I-140 approved in 200 days then you can use AC21 rule on 201 day.(Next day after your I-140 approval, if it crosses more than 180 days)
My guess is that when they published the AC21 rule, there were no concurrent filing and approved I-140 was the basic eligibility for using AC21 rule.
Again these are just my guessings only. Consult with your attorney for advise.
Nanshi.