Now that I can copy and paste, don't ask why, here is my opinion.
Low 6-B/6-B Criticism
Okay, first of all, who tanked Takemigatsuchi's beam anyway? No one did. Even if we count Rachel, she did it with Tsukuyomi Unit, that doesn't count. Mu-12 only survived by cocoon, and even then she get damaged. Not just that cocoon is non-combat applicable and not scale to herself, she doesn't have it in the first place. It won't count either. Why would it be the potential feat to be scaled if no one can or have reason to be scaled by that shot? No one actually killed Takemigatsuchi in his weakened state. Or fought him. Nu-13 only interacted with him once he's freed. And she didn't even fought him.
I suggested Low 6-B because of the plot of Variable Heart, when they had an ability to smelt Takemigatsuchi out of them, so they should have half of power, which is 5.6 Teratons, then scale to Mid-tier. The problem is, we treat Takemigatsuchi for some reason, to be Low 5-B even if I disagreed. Mai and Platinum feat is now not the same, they're now half of Izanami's 7 zettaton feat, which make them 3.5 Zettatons. Mid-tier become Low 5-B. It's insane. If we scale this way, Mid-tier now merge to High-tier, it doesn't even need to be called Mid-tier anymore. Another route is scale it by Nine, but Nine is High-tier so there's that. I see no other way to scale it to any Mid-tier ever.
Low 5-B Black Beast and Takemigatsuchi is a bad idea
Someone might suggest that Low 5-B Mai should be a thing, which isn't totally should, this will turn Calamity Trigger Ragna comparable to Post-Idea Engine Ragna, almost comparable to Chronophantasma Ragna, etc. This also undermine the idea to not scale every character to one feat, that's why Matthew did that one thread about it anyway. This also makes several plot holes and assumption as well. Like how was the Takemigatsuchi's blast threat to all of them if they can totally tank it, how was Mu-12 without cocoon killed by Takemigatsuchi's blast if she's Low 5-B in Ibukido and Full-powered Azrael and 100% will be stronger than Susano'o and Izanami by multipliers. Which is pretty consistent btw. You know, all of this.
Now, it's impossible for Takemigatsuchi to not scale to Black Beast. At the end of 2107, when the Beast returned, Seven and Eight called Takemikazuchi on the battlefield, and managed to take some power from Celica, enough to make just one shot that made the Beast to escape from there. They have history and they have feats to back it up. Not to mention Takemigatsuchi is specifically made to take down Black Beast. Also yeah, that statement means Takemigatsuchi barely had magic element.
Black Beast to Izanami however, was never shown. Black Beast never fought Izanami, at least I can't recall it. Also can we talk about scaling we have to make when we make it Low 5-B aside from Takemigatsuchi problem? Calamity Trigger Ragna, aside from scaling from Variable Heart Mai, will scale to 20% Hakumen, who scales to Terumi, who fought against Black Beast, who fought 100% Hakumen? And comparable to him? Low 5-B Ragna at very start? Full 7 zettaton at that? Hakumen thing also doesn't make sense. The fight is decisive so Black Beast being 6-B while Hakumen and possibly Jubei being Low 5-B makes some sense. This also doesn't create some really confusing scaling. Again, it was never shown, only said. It can totally be hyperbole if not outlier.
Solutio
The best I can think and approve is Black Beast being 6-B, not Low 5-B. It will create the scaling some cohesion. This will also make 6-B feat scalable. Basically creating a Mid-tier possible. Six Heroes besides Hakumen and Terumi will be At least 6-B, possibly Low 5-B. Terumi will get a power escalation. Central Fiction Terumi is superior to his Continuum Shift self after all, he fought Ragna at the Blue who can hold off Susano'o while he got ****** by Post-Idea Engine Ragna at Continuum Shift.
Second option which is lot more extreme, basically ignoring the Mai feat as outlier and dropping everyone who doesn't scale to Low 5-B to 7-C. Yes, it includes Restricted Azrael, Kagura and Hazama. It's downplay but at the same time, it's a safe bet.
The third option is accept Low 5-B Mai. I told you guys why it ain't an option. It will be a mess.