But hard for whom? That's the question, isn't it? Do we set difficulties for:
"Hard" for someone who is "good", that was in the OP.
You have to decide for
yourself (which is why I left it open-ended) what constitutes "good" (what modifier total) and what DC would be "hard" for that individual. Once you make those choices, you get the % chance and vote.
Now, you can do it the other direction as well:
For instance, given for most PCs the range is -1 (ability score 8) to +17 (ability score 20, tier 4, expertise), you might decide someone is "good" at +8. They aren't great (maybe +10-12) or incredible (+13 or whatever...), and they are better than someone who is decent (maybe just +5?).
It is entirely subjective, of course.
Then, you think: "How likely should someone who is
good succeed at a difficult task?" Perhaps 35%? 50%? 65%? 90%?
Again, entirely subjective and you have to decide for yourself.
Doing a bit of algebra will then decide the DC. In the example above, if you are +8 and want a 35% chance of success. Well, 35% is a roll of 14 or better, adding 8 makes the DC 22. Hmm... is that too high? Maybe, maybe not...
50% would be DC 19, 65% would be DC 16, and 90% would be just DC 11!
So, what seems most "right" to you? Is "good" +8? Is "difficult" DC 22, 19, 16, or 11? And of course, ultimately, what are then their chances?
You can go as in-depth in your thinking or just go with your gut on what feels like the chance should be for someone good doing something difficult.