"Optimum" often varies depending on whether it your own opinion of your performance or other peoples...
As an instrumentalist, I suspect that optimum levels are lower that for singers. I often don't get through more that one pint at a gig because there isn't so much time to drink it, and I don't drink much more than that anyway because I'm usually driving.
The thread title causght my eye, though, because I was recently introduced to the concept of the "grumpy pint". When someone reaches their "grumpy pint" level, you'd better get them another one quickly...
Anyone else got a known grumpy pint? I'm uncalibrated so far...