For me as beginner this explanation was very helpful. Thanks.
Also how to check the optimation on Thoughts on optimizing gain - #2 by wiedehopf was interesting.
But for impatient people waiting after each change at least 5 minutes are quite long. So I found a workaround, not very exact, quick and dirty.
First, check the actual 1090 data, for example http://your-adsb-ip/dump1090/ or http://your-adsb-ip/tar1090/ and note the number of planes.
Then change the values, reload the page and re-check the number of planes.
The difference is a good indice if a optimation is necessesary and if you move in the right direction.
In my case with -10 gain I saw around 80 planes, with 30 gain there were only 4 planes left. Also gain 20, 10 and 0 marked less planes on the 1090 page. With -10 it was okay again.
So no optimation is necessary.
It looks that with a wrong optimation the results are even worse, so it is not necessary for all installations.