Hiya!
I can get those results without fudging dice by cooking the stats going in. Blaming fudging dice isn’t a solution if that’s my attitude.
A far better way to allow a recurring villain escape is to compensate the players for it. Giving out hero points in Mutants and Masterminds is a pretty good method. An equivalent in 5e might be giving everyone inspiration for every DM-fiat recurring villain escape from doom.
IMNSHO, "cooking the stats" isn't any different. The DM is still pre-emptively 'deciding' the results...although to a perhaps lesser degree. Now, add in fudging on top of that...well, you've only compounded the error. This is the opposition I have to "building the encounter to fit the PC's". It is effectively the DM "stacking the deck" in favour of, or disfavour of, the Players success. That just rubs ME the wrong way...if it works for some other group, fine, different strokes and all that, right?
The best way to get a recurring villain is to let it happen naturally, without the DM's "input", so to say.
Now, don't get me wrong. A DM that makes a dungeon filled with CR10 monsters for his groups party of 3rd level PC's is failing MISERABLY at the "be fair and neutral, neither favouring the Player Characters or the Monsters". For me, when I'm stocking a dungeon or writing out a wilderness area, or tossing together an adventure...I do use the very minimums of the group's PC's. It's basically "How many PC's, what's the average level". That's it though. I don't care what races, classes, skills, magic items, spells, equipment, etc that they have. Then I 'build' the dungeon/adventure around the backdrop story, the location in regards to it's place in the world, it's history of who/what/where/when/why/how, and off I go! Like any writer, at some point the thing you are writing takes on a life of it's own; sometimes it "writes itself" (ask any fiction writer and they will often tell you that at some point, a character in their story sort of 'tells them what to write'; same sort of thing with a dungeon complex, or series of ruins, or whatever.
So that's how I write. To the "world and campaign"....the PC's are an afterthought, to be honest. Every now and then I MAY toss in something I think is cool, makes sense, and just so happens to tie into a PC's bonus/penalty...but I don't do it the other way (writing something in because of a PC's bonus/penalty).
Anyway, this is getting a bit off the core topic of BBEG combat length. So I'll leave it at that.
^_^
Paul L. Ming