I personally believe that it is nearly irrelevant how NPC's compare to each other in power and levels. All that matters is how the NPC's compare to the PC's.When I work on my setting, I always try to remember "all those city guards and thugs are 1st level warriors, with gate guards at the castle and highwaymen maybe being 2nd level fighters". Everything else is based on how the new NPC I am working on would fare against such opponents.
If the character could not beat up a partrol of 10 regular guards without any real danger, then he's probably only 4th level or lower.
"D&D E6" didn't get you a result?
Though in fairness, the material only exist in a forum thread and does not have a website, that may be a bit confusing:
http://www.enworld.org/forum/general-rpg-discussion/206323-e6-game-inside-d-d.html
I personally believe that it is nearly irrelevant how NPC's compare to each other in power and levels. All that matters is how the NPC's compare to the PC's.
So assumed the party of four 5th level PCs is lead to the lord of the local duchy and they encounter something like this:
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A cool boss fight for the party would be a 7th level NPC, but would that really be appropriate here? From the look of it, if she's alone on the street at night and two guys with knives attack her, she's probably very dead. Even if she does happen to have a black belt in Judo and catch them by suprise, she's still in very serious danger. So I would make her a 2nd or maybe 3rd level expert, not a 7th level rogue or sorcerer.
And that means if the PCs would want to kill her, and I want to make it a big fight, then the fight would be with her bodyguard, which may very well be four fighters of 3rd or 4th level, making a nice EL 7 fight.
On the other hand, assuming the PCs are 8th level and walk into the throne room with full armor and weapons, they practically have both the duchess plus her four bodyguards, and all the other 1st and 2nd level retainers as hostages, like the Joker on that fancy upper class party.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.