Let me elaborate on my post. This is based on 10 of your best 20 scores. This is also based on playing the proper tees for your skill level.
Finally, this is the terminology I would use. My index is 6.0 so depending on the course rating, I shoot anywhere from 75-85.
I consider myself a “good to pretty good golfer” if someone asks me. Probably needed one more range: breaks 70 in tournaments as “phenomenal golfer”. A guy/girl who wins the US Open and a guy/girl who wins the US Amateur or both phenomenal in my opinion but obviously one is way better than the other.