Games vary pretty radically in how combat works, and part of the advantage of PCs is they can come up with ingenious plans to get out of tight scrapes. Thus. rater than trying to come up with a mathematical system, I use movie logic instead. Its pretty rare in any media to get the main event first. Test them with lesser fights and ramp up to what they seem capable.

Generally speaking, enemies that are too weak might be uninteresting, but far more entertaining that the process of making new characters. Making up a reason for a last minuet save is quite a bit more contrived and story breaking than reasons for an understrength enemy.

Maybe they just come across a few stranglers of a road march, or the advance patrol. Perhaps someone is simply testing them, or the enemy is over confident and holding back.

Additionally, you can plausibly reinforce a weak encounter - someone heard the fight, they were searching for their lost friends etc. A winning opponent simply abandoning the fight would be rather unusual.

There is a fine line between hobby and obsession. I seem to have lost sight of it some time ago.