Rx Noise Indicator.

Installed WSPR 2 yesterday. So while I was at it I decided to read the instructions! What perplexes me is the Rx noise indicator. The instructions tell me to try to reduce that to as close to zero as possible using the RF gain and AF input volume to the computer. Neither adjustment makes a blind bit of difference to the Rx noise. It's always -30 Db. Never changes, apart from when in Tx mode of course.

Have I read the instructions wrongly?