What ranges?
This is what I wrote.
The range you can hit 5 out of 5 shots is your limit. NO MISSES AT ALL!
How many plates?
I would say one plate for every position.
1. Prone, 2. sitting, 3. kneeling and 4. off-hand. So 4 plates.
Shoot at your first range. If you hit it 5 times, back up 50 or 100 years and do it again.
Keep backing up until you miss.
That's you you know what YOUR range is. With your gun from that shooting position.
I come at it a little differently. Shooting prone off a backpack or bipod at the range doesn't mean you can do it at the same distance when you're on a snowy slope with a sharp rock under your crotch and the wind making your eyes water.
So I don't put much stock in "I can hit at 500 yards at the range 5 out of 5."
If you shoot enough, when you get in position and put the crosshairs on the target and assess the wind, light, and everything else, you'll know whether you should pull the trigger. Key words are "shoot enough."
There is no substitute. And no magical yardage.
Except to say that if you can't hit at a particular distance with good conditions at the range, that's obviously outside your envelope.
