But why not stay closer to 3e, 4e and 5e CRs, and then let the number of mythic stages be the upscaling factor, like you considered in an earlier post?
Edit: myself I'm thinking a base CR of the 4e CR minus 5 (except Gods) . So Demogorgon CR29, Tiamat CR30,Orcus CR28, Ancient Red Dragon CR25...