That's not actually something that can be boiled down to anything near concrete - really can't even get as close as the already very loose encounter building guidelines, and I'm not just saying that because there is currently no such thing as a CR31 creature.
With class levels up to 20, there are predictable gains, and while they are not entirely equal at every point there is a shared framework that isn't severely deviated from regarding how many combat, non-combat, or dual-purpose features each class will get.
Once you get to the epic boons, there is no more predictability - one character might gain Immortality, which doesn't actually change their ability to overcome challenges outside of the very narrow band of things which might age you to death, and another character might gain Skill Proficiency which provides a significant amount of ability to overcome challenges in all pillars of game-play, and another character still might gain High Magic which is even more potent an increase in ability to overcome challenges.
So, in my opinion, the best way to handle challenge estimation once you've reached the 20th+boons level of play is to not actually try to get a close estimation in the first place. Anything CR 20 or lower, unless encountered in significant numbers, is fully within the realm of expecting the party to be able to handle. Anything CR 21-30, the party can probably also handle unless found in numbers or with a lot of lesser creatures aiding them, they just have to be a touch more careful in selecting their strategy.
I haven't really tested the theory yet, but I expect that there isn't a point at which CR 21-30 creatures become non-challenging to the characters just because they've accumulated X number of epic boons, since even the benefits of epic boons are limited (ability scores don't go past 30, and each boon can only be taken once unless it specifies otherwise, which only the option to increase an ability score actually does).