After looking into the scale of the Measurement chart there is little reason to go beyond 30 as a base even for upper cosmic types. So when you see Monitor, Anti-Monitor, Galactus, and all manner of others in between (like Lords of Chaos or Law), they have stats that fall largely from 20 to 30.
I say that based on the soft cap I have seen and the way characters have been stated. One major way to help determine the stats is to go by the DC you wish to achieve. Since DC 40 is 'near impossible' and one assumes 45 would represent ímpossible' results (though if I had made the chart it would have been designed so that 50 were impossible, but as it stands as written 45 is the number)
Knowing that, you have characters like Mr. Myx or Impossible Man who can routinely warp reality, that is ímpossible normally, but if we state that doing so is simply achieving ímpossible results on a DC check, whereas ''near impossible' would be inventing time machines and whatnot. Then 25 + 15 = 40, and a +25 bonus can do that. Seeing the stats then extend from 20 to 25 would be for most cosmic types. Galactus for example, if he had a 24 Power Cosmic would be able to function like you see in the comics and still be clearly one step (+5) above even someone like Superman. Above that might be truly cosmic powers like Death or Eternity but you could place them somewhere between 26 to 30. You could just go with 30 as a 'soft cap' and thus place the beyonder, if not at 30, place him clearly above even those cosmic types at 35 to 40, so 35 probably works. Yet, I would tink if someone has +30 +15 = 45 you can do 'near impossible' before you even roll, and say 10 is a routine check, you'll achieve a DC 50 by simply getting an Easy result.
To me, using the DC then helps determine these high end characters. I have a modified chart so that 'impossible' = DC 50 and the chart starts at DC 5 (since 0 is now Automatic, since you don't really roll), but I didn't design the D20 structure itself