HOWTO: gain
This guide assumes you have console access to your Raspberry Pi
(For Beginners - How-to SSH to RPi - Setup Putty in Windows)
and know how to change the gain.
(For Beginners - How to Set / Change Gain)
Use this command to get the percentage of strong messages:
awk "$(cat /run/dump1090*/stats.json| grep total | sed 's/.*accepted":\[\([0-9]*\).*strong_signals":\([0-9]*\).*/BEGIN {printf "\\nPercentage of strong messages: %.3f \\n" , \2 * 100 \/ \1}/')"
Percentages around 5 are what you should be aiming for.
If you live close to an airport even 10% or 15% can be an optimal value.
If you have very occasional helicopters / small plane that fly close by but most other traffic is far away you might need to go for 0.5 % or even lower to make sure the close by traffic is received.
This is just a guideline, experimenting until you actual start losing close by planes due to overloaded receiver is encouraged.
Now you adjust the gain.
After waiting at least 5 minutes you can check the percentage again using the command mentioned before.
If the percentage is too high for your taste, reduce the gain, if it is too low increase it.
If you are unsure of what you want just aim for 5 percent 
Repeat until the number is to your liking.
If you don’t care about losing messages from planes that are close to you and are just aiming for maximum range then you can go higher but i wouldn’t recommend it, you might not get any extra range by using a higher gain. (The noise is amplified with the signal, so if the noise is “louder” than the signal no amount of gain will make you receive that signal)
If most planes are far away and flying high and every few days a helicopter flies by very close and you lose it when it comes closer you will need to reduce the gain to accomodate seeing that helicopter and the percentages will be quite meaningless.
After you are done you can check back after a day or two to get a more precise percentage again using the command. The percentage is only reset when dump1090 is restarted.