I'm not sure why a 1-2 second fade in/out of the tone would not be better than starting it abruptly, regardless of where in the waveform it starts. I'd be curious to know if I'm missing something in that regard.
It has been some time since I did this, but I'm pretty sure the headphones weren't off by more than 10db at any frequency I was measuring.
The vertical scale is db, but it's relative db. I think one unit is 5db, but I'm not 100% certain.