I have this idea that one day, in the far, far future, we will no longer need to say "do what works for your group" because we'll know that has always been true
Anyway, what I was drawing attention to is that DCs appear to be intended to be objective, not subjective. That seemed to mean focus on the jump distance, not the jumper. When I do that, it feels easy to reach a consistent ruling. The 18 Strength character never has any issue clearing 10', whereas the 10 Strength character finds it a real challenge to clear 18'. Characters, generally, are challenged to clear 20', but higher level characters will find that more doable.
I then had the further thought that perhaps for some interpreters, the amount of increase over a character's usual jump constitutes an objective fact about an unusual jump. The objective part of the DC setting exercise is determined by looking at the specific character, and the subjective part of the DC setting exercise is determined by looking at the specific character. The aim, I think, is that bringing the character in on both sides means that clearing 2' or 20% more or whatever distance over your usual, has the same difficulty for all characters that attempt it. I then hit a standard problem with flat adds, which is they don't respect the diversity of features that can mean for some characters, an extra 2' is almost nothing, while for others (in an extreme instance) it nearly doubles their jump!
In terms of the question - is this what the rules guide to doing. I think they don't, because I think the character is not meant to come in on both sides of the DC setting exercise.
In terms of the question - does this work well? I think if you go this route, percentage makes far more sense than a flat add. I guess one chooses a scale - say DC 5 for each +20%. The math then is pretty straightforward - I can usually clear 18, I want to clear 25... actually, the math isn't super-straightforward. No wait, it's +40%. DC 10? I might need to roll 4+. Say I go for 35', about double, DC 25, I might need to roll 19+. Clearing 40' is my maximum without Guidance or Bardic Inspiration.
And there's the rub: players have all kinds of resources to bear on this, that can make a high DC doable, or circumvent it all together (cast Fly?) That makes scaling subjectively for me a questionable pursuit, but if I did scale, I'd scale percentually.
Again, what works for your game is just friggin' fantastically doodlely wonderful.
For my games, taking a task like say jump (or say another strength task like say lifting) where the basic "what you can do fine with no risk, no action, no trouble" is entirely varying per individual and keyed to strength... taking that task and then saying "but now i will set the DC for "a little bit more" and totally divorce it from the "same thing it would be based on if it were 2' shorter" produces rather inconsistent results which lets weaker characters do "a little better than normal" with a higher confidence than the stronger more athletic one does.
It would be as if i were staging a drinking contest - drink 10 flagons of ale - and had the halfling, the human and the huge giant face the same DC because i wans't going to adjust for their size.
its like say - to go to a more strength related point - if i had a halfling loaded to his carry capacity at str -8 and a brawler str-18 prof athletics loaded to his str 18 capacity and i wanted each to carry 2lb more. Were i to assess a DC for "fatigue" after a while of lugging that around because you were overloaded, it would not be based on the whole weight carried (the acceptable no problem weight and the amount of extra weight) as a total DC. Most folks would think "the extra 10lbs on top of 240 is practically nothing, but the extra 10lbs on top of 120 might be noticeable as far as long term fatigue would seem. (Both can carry encumbrance penalties of course and again we see those scaling based on "the safe weight/strength and how much over - not a total factor.)
But sure, for jumping, any Gm can rule that unlike a bit of the other strength stuff its gonna be based on a total not the overage compared to the baseline and thats scrumdiddlicious for games where that kind of thing makes sense.
its all good.