I don't quite understand why you are locked into the encounter math as the solution to your problem.
It seems to me that satisfaction for a deadly encounter boils down to the following:
Players - they were fearful that at least one of them could potentially die, but they escaped by being smart or lucky
DM - the encounter give the players enough of a scare to be worth the XP granted
[MENTION=6787650]Hemlock[/MENTION] said it well. It's about getting that feel as a DM for what a group can handle. I certainly don't think math is the only part of that, but I do think even in 5e that it's a significant part.
I did some thinking about why, personally, I'm looking at the math as significant....
Part of me has a sense of "fair play" as a DM in that I won't exceed the deadly encounter guidelines unless I give my players foreshadowing or they clearly bring it on themselves (either thru a colossal Bad Idea or by deliberately seeking out a challenge they know is super-deadly). Maybe it's some kind of "gentleman's agreement"? Even back in AD&D I DMed that way. And back in AD&D there were even less encounter guidelines and I just developed a feel for what my small group of friends could handle when it came to combat. Of course, there were several "learning curve" disasters along the way as I figured out what was too easy and what was over-the-top too hard.
I'm re-experiencing that a bit with 5th edition. The ground, so to speak, feels shaky under my feet.
Obviously I've got many more years of experience to draw upon now, so my "learning curve" mistakes are
probably not going to be disasters, but there's still very much a sense of "I don't know if this is going to challenge them the way I think it will!"
Maybe that's OK?
Maybe by the time we hit 5th level I'll have developed a good enough instinct for it that I won't even need to consult the DMG?
Maybe somewhere along the way I'll throw out my "gentlemen's agreement" and just throw whatever the hell suits the narrative and my own fiendish whim at them?
However, regardless, I think we can all agree that the ability for the DM to predict "this is a deadly fight" is useful to have in our toolboxes, right? For world-building purposes, at the very least.
You said that one player was KO'ed, but got back up and nobody was worried, so why not house rule so that they are worried?
If a PC drops to 0 hp, grant them a level of exhaustion and have them roll on a lingering wounds table - now the players are scared for at least one PC.
Aren't your three carrion crawlers deadly now?
Players are not in favor of that. We talked about house rules and such at the start of the game. Because we have a mix of veteran players from older editions and a few players brand new to D&D, and pretty much everyone is new to 5e, we decided to minimize house rules to ease everyone's learning curve.
The only house rule that everyone was excited about, oddly enough, is critical failures. They came up with a whole table of crit fails options, if you can believe, of their own prompting!