5m is useless.   20m is reasonably accurate but certainly not 100%.

In other words, there's no hard fast percentage.  

Correct.   If there was, say 20m was 90% accurate and 10m was 45%, you could budget 2m and figure it out using simple math.

Thanks, I was afraid that would be the case. I was hoping there was some way to determine a range. For example, if a $10M budget was accurate within 10 points, and I had a guy with projected 80 power, I'd know he was really going to end up somewhere between 70 and 90, but that he'd never hit 95. I was hoping there was some sort of rule of thumb like that.