In this case, using a Signalink USB, the best procedure to control the signal level is to use the Signalink controls. The Windows CODEC should be set, and left, at 0 dB. Note that the default Windows scale is percentage, right click on it, and change to dB. Never change this, always 0dB. As for the signal levels WSJT-X 'calculates', it always the lowest level measured during the 15 seconds cycle. This is why it changes so much, or shows a low reading with a strong inprint on the waterfall. I've been a big user of JT65 since 2010. As a matter of fact, thanks to it, I have completed awards that I doubt very much would have been possible with my modest station. That said, I have to say that I find JT65 unbearably slow now.