Define "relatively good"
To me good is being able to break 80 any and all the time. In tournaments, on different courses from the tips. It's not "elite" but it's not embarrassing either, even playing with pros. Basically, a few times under par a year with most scores in the 74-77 range. Really about as good as an adult who picked up the game can hope to be.
I think that would be the baseline for a good player Mike. The caveat being that the 3-5 index is more of a tournament index.
As for the courses, sure, if your playing a 78.1/152 course all the time then breaking 80 would basically put you at scratch. Still I think breaking 80 is a reasonable goal for 99% of courses.