I don't think the exact formula has ever been spilled. But from all my observations here's my best guess:
If your guild is on RFH crypt each member contributes 1.5 (round up) lines to crypt. I assume it sorts by team power and uses the same opponent selection methods/pool (FP) as Expedition. Power is the only relevant comparison performed.
So crypt line #1 is roughly equivalent (if not exactly) Expedition 2.1, 3.1, 4.1 for your guildmate w the highest team power.
Crypt line #2 is Player1's Expedition 2.2, 3.2, 4.2 or something very close to that.
Then Crypt line #3 is Player2's 2.1, 3.1, 4.1
Crypt line #4 = Player3's 2.1, 3.1, 4.1
Crypt line #5 = Player3's 2.2, 3.2, 4.2
and so on, alternating one or two lines based on each members' team power.
With the recent level cap bump Exped lines have become worse than usual. Many of us have noticed these anomalies in Exped Stage 5 forever-ish. But right now they are showing up in 4.1 coincident w their appearance in crypt.
What exactly causes these strange lines in exped is still a mystery. But it certainly seems to affect players near the top of the power scales. Normally Stage 5 is supposed to be above your team power. So what exactly does happen when that's not possible? While a wrap-around idea seems close, it doesn't quite fit observations exactly. If that were the case stage 5.3 would be the most likely to wrap and 150k would become 10k. It doesn't quite do that. But in practice the first to "wrap" is 5.1. Followed by 5.2, 5.3, 4.1, (4.2...? I've not managed to push it that far personally). It seems to try to drop things earlier by one stage. Presumably an intentional difficulty modifier. So what would have been 2.1 shifts into 1.1's spot. 2.1<=3.1, 3.1<=4.1, 5.1<=(1.1?) since there's no 6.1 to draw from.
For reference I'm somewhere around #1,000 in team power (varies alot) on my server. Before cap bump, my entire Stage 5 was easier than 1.1. When cap first went up, the top hundred or so really push that envelope stretching out the scale. My 5.3 became a "real" line again indicating I'd lost one stage of wrap relative to the community extremes. Since then I've spent alot of stamina items and pushed to 125 and brought several heroes to o8 adding alot to my theoretical max team. I'm now getting wrapped lines to 4.1. And being about 8th in my guild we're getting a few of these odd balls in crypt daily.
As I recall, something very similar happened last cap increase. But that was so long ago. But it settled out again after a few weeks iirc.