Your bland doorman has a stress track of length 2. A mild consequence brings a hit down by 2; a moderate brings it down by 4. In order to get a taken out result, you need to land a blow that inflicts more stress than the stress track can handle. Without consequences reducing the hit, and no roll-up factor due to boxes on the track previously being marked off, that's 3.
Now, this dude's a doorman, so as the GM I'd probably run this as "if a blow is going to inflict any consequence at all, even a mild one, I'll just offer the concession of being knocked out". So, a hit that inflicts 3 stress would do that.
But let's say I'm buying your proposition -- the "player" (the GM) here doesn't want this character to have a lasting issue, which starts at moderate consequences since you can "shake off" a mild with a scene or so of rest. In that case we're looking at a blow that needs to land 5 stress -- 5 because if you reduce that by a mild (2), you're at 3, which is still beyond the end of this guy's stress track; he'd have to take a moderate to reduce that to a number that lands on the stress track.
Rolls will come into this, but let's "take zero" and assume everyone performs right at their skill level. With him having Mediocre Fists, if your PC has at least Good Fists the margin's gonna be 3 on average, which is enough to inflict a mild consequence; if moderate is what's required in the GM's mind to force a concession, then your PC's gonna need Superb Fists.
But...
Your PC might have an applicable stunt, boosting the effective Fists rating here.
Or a fate point and an applicable aspect.
Or he's taken the previous action to roll Stealth to set up a temporary aspect on the target, "I've Got The Drop On Him" which he'll be able to tag (at no expense) when he swings his Fists.
Or maybe you're playing it loose and clear enough that this guy's really just a difficulty number, not a full blown character where you'll need to engage the stress track at all -- just set the difficulty at something reasonable to get past the "obstacle".
Or maybe you're playing it loose and clear enough that this guy will go down with any solid rationale, and spending that tag on the temporary aspect you got with Stealth in a "compel for effect" way is enough to say "How about he just folds long enough for me to get past him and inside with a couple minutes to spare?"