Okay, here's my NTRP guidelines. I think I've done a pretty good job of coming up with practices that are more specific than "Being able to hit with spin." Let me know what you think
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You get one point for each of the following that describes you or your capabilities:
Your first serve is a weapon that can often force errors, even against better opponents.
Your first serve has enough variety that you can keep your opponent guessing about placement and/or spin.
Your second serve is a "real" second serve, with either a lot of pace or so much junk on it, you can force a good number of errors.
You can often (but not always) take advantage of opponent's mistakes, such as standing in no-man's land, weak approach shots, balls bouncing into your strike zone and overly easy second serves.
You have good, solid passing shot and lob capabilities and can hit these even when pressed by a good approach shot.
You miss less than 10% of your overheads.
You're as good at knowing what shots NOT to hit as you are at what to hit. (For example, you don't try to launch forehand winners from a shot that's just above your shoe laces. Bonus points if you CAN consistently do that)
You can often turn a body shot at the net into a volley winner.
You're good from BOTH sides of your body and your opponent can't exploit a weak side.
You can quickly read your opponent's weaknesses and exploit them consistently.
You hit hard enough that you have to replace your strings often and/or use heavy duties.
You relish your opponent's drop shots because you're getting to 95% of them. Your opponents are often demoralized when back-to-back "winners" are returned. Hard.
Your (reputable) coach says you're a blank point blank . (This can bump you to the next grade if you need just ONE point.)
Scores:
1-2: You're a 2.5
3-6: 3.0
7-9: 3.5
9-11: 4.0
12-13: 4.5+
Of course, many of these are RELATIVE. Which makes it even harder to figure out what you are until you play a half-dozen or so ranked opponents.