so far I have noticed that when my CNS is fresh and on , I can tap 19 (sometimes 20 - once so far) in 2 secs on Taptester. and about 85 on 10secs
When I am tired - 18 is the most I can get no matter how hard I try and 75-80 on 10secs . So I suspect if my CNS is really fried then I won't be able to crack 17 and 70.
When I get 18 I can still have a good workout and set PRs, but I don't feel fresh like I do when I hit 19-20.