I myself treat a character with 6 aspects, 3 stunts, and a pyramid that caps at +3 as a 1-2 lvl character; 8 aspects, 4 stunts, and a +4 pyramid as 5th level, 10 aspects, 5 stunts, and a +5 pyramid as a 9th level character... though the arrival of 4e has meant that I sorta have to rejigger what counts as "paragon".
I still haven't decided how I want to extend SotC into epic, if indeed I do
So considering that D&D has a "randomizer range" of 20 (1d20) and SotF/FATE has a randomizer range of 9 (+4 to -4), it seems like each unit in D&D should equal 1/2 unit in SotC, or really close there. Especially in editions earlier than 4e, where a single hit can kill you, the condition track sorta backs this up if you count each box as a unit too.
So in this case, 1st to 2nd level should be pyramid of 1, 2 aspects, a stunt, and a single conditoin track box. For every 2 D&D levels, you increase the pyramid by 1, give another 2 aspects, another stunt, and one more condition track box. Standard SotC would be ~10th level, which sounds about right for paragon level Century Club members.
So my partial hack so far has, divide D&D level by 2, round up. Set aspects, pyramid, stunts, and condition track to this number. This is fewer aspects than standard SotC, but I also allow 2 open story aspects that you can fill in in the middle of play to reflect the current conditions of the storyline, and I also allow on the fly stunts. These aspects and stunts give are sorta the basis for what they gain as they level.
Things get sorta wacky above 10th level for pre-4e editions, but that is when they always did, hence the E6 hack popularity. The stunts to simulate D&D spells are being worked on. I replace BAB/THACO/Fighty ability with Melee and Ranged skill, and magic ability with a magic skill and some sort of magic aspects. Weapons are bing simplified to unarmed/small/medium/two handed for -1/0/+1/+2 to hit. Armor is being simplified similarly. Magic items give aspects, but not stand alone bonuses.
I am also playing with a series of free aspects and stunts that come as a package at chargen to sort of simulate class. These will be along the lines of striker/leader/controller/defender from 4e, with the stunts designed to allow eack character to pull off that role, no matter what skills are taken to support it. Something like controllers get a bonus to adding aspects to enemies, defenders get a bonus to block actions, strikers obviously do a little more damage, leaders can buff/heal in some fashion. Multiclassing is handled by taking aspects/stunts. So a paladin might be a defender, with extra aspects dedicated to religious stuff and stunts dedicated to "leader type healing ability". So it basically is classless, but there is a free package that everyone gets that simulates classes.
I also like using generic stunts. +2 to specific, +1 to general, substitute skills, or do something not elsewhere covered in the rules. Players can come up with anything that fits this model. Magic is being workded on.
The hardest part is that I need to playtest the Shadow of Yesterday keys hack that I am doing for XP. I think that I just need to play, give XP according to the keys, and then level up the party when I feel it is time. The amount of XP that they have at that point should be what it takes to level for future games.