Step 1
First step is to install Python on your raspberry Pi
sudo apt install python
Step 2, create a blank file named optimize-gain.py
(2) Create a blank file:
sudo nano optimize-gain.py
Step 3
Copy and paste script 2 from this post into the blank file
Step 4
Make sure you copied all the lines 
Save file Ctl+o and exit editor Ctl+x
Step 5 Make file executable.
sudo chmod +x optimize-gain.py
step 6 Run the script.
sudo python optimize-gain.py
Step 7
Go to kitchen and prepare a cup or two of coffee. You will need it during the long wait the script is running and outputs the final results. 
steps above are from @abcd567 , credit is towards him for this instruction.
This test wil run 5 times the same values, this will give you an insight in the total values for the different gain steps.
You will have one value with the highest postion count, maybe the same or another with the highest plane count.
It is then up to you to decide to go for the postions or the number of planes.
Set that gain number then as fixed in your setup.
Edit the dump1090-fa config file:
sudo nano /etc/default/dump1090-fa
Look for the following parameters
set ADAPTIVE_DYNAMIC_RANGE= from yes to no
set RECEIVER_GAIN=60 to the desired gain value ( use a point and not a comma when implementing decimal values)
Save the file
Restart dump1090-fa to have the setting take effect
sudo systemctl restart dump1090-fa
During the measurement your stats on graphs1090 will be messed up, this due to the fact that for every new gain setting the dump-1090-fa proces is restarted.
Note, this takes some time, average time on each pass is 15 minutes so it wil take you 1,5 hrs in total to complete the test. Be patient while the script goes through its moves.
In the end it wil display the weighted result of the 5 passes 