Using sites like Radarbox etc are not a good solution finding the best gain.
I would suggest checking with the method @wiedehopf documented on his wiki pages as suggested already.
Optimizing gain · wiedehopf/adsb-scripts Wiki (github.com)
Or this mega thread:
Thoughts on optimizing gain - FlightAware / ADS-B Flight Tracking - FlightAware Discussions
I am always doing it manually and not using the auto gain feature of dump1090.
A while ago i am using readsb only and set gain manually. I find it better than dump even if you have to manually set gain. But i am normally not a fan of too much automatism
Beside checking the graphs there is also a short command line in the wiki article above where you can check the values without waiting for too long.