This will be a mini howto about custom frames (like my SG Adventure Mini) and the motor mixer settings for different flight controllers. I already described a bit about the settings on KK2.0, KK2.1.5 or the Naze32.
There are two methods about getting the right numbers for your flight controller. The first one is by using the angles between the arms and the roll and pitch axes. The second one is by getting the distance from every motor to the Roll and Pitch axes. I used the first one for setting the mixer editor on my KK2.0 board about two months ago, because I saw this method in a HobbyKingvideo on YouTube.
Although I will use the second one in the future because it is more accurate. Why?
Using the first method we use the angles between the motor arm (or in a bit asymmetric frame the line between the motorshaft and the center of where the flight controller is mounted) and the roll and pitch axes. Then use the sine(of this angles) = the value we use for our flight controller. This is actually almost the same as method two:
For the angle α, the sine function gives the ratio of the length of the opposite side (the distance from the motor shaft to Roll/Pitch) to the length of the hypotenuse (this is the distance from motor shaft to the center of the flight controller).The controller uses only the ratio between these two sine functions and in that way it knows the ratio between the distances to Roll and Pitch and where the motors in space are.
Using the method with the sine of the angles would be 100% ok, if all 4 arms have the same length and are mounted in the same distance from the flight controller. On my SG Adventure Mini Quad the distance from the front two motors to the FC are 135.3mm and the distance from the rear motors is 136.2mm. That is less a mm. Using the method with the angles would be no problem at all.
To be 1000000000% accurate 🙂 I am using the second method, getting the ratios of every distance to the Roll and Axes by measuring these. How to do this:
Measure all the distance from every motor to the Roll and Pitch axes. Take the biggest nominal value and divide all the distances by it. In that case every distance is a ratio of the biggest one.
In my case this is the distance from Motor 1 (Rear right) to the Pitch = 108mm. Dividing:
Motor 1:

Pitch 108/108 = 1 (For the KK2.0 or KK2.1.5 Motor Mixer this means 100).

Roll 83/108 = 0.768 (KK boards 77)
Motor 2:

Pitch 83/108 = 0.768

Roll 106,9/108 = 0,989
And so on for the other motors. Note that the motor layout in MultiWii/Naze32 are not the same as in the KK or the APM.
And to show you, that there are (almost) no difference in the calculation methods:
Sine(38) / Sine(52) = 0,782; 38 and 52 are the angles to Pitch and Roll for Motor 2 on my frame.
0.768 / 0.989 = 0,776; This difference is so small, that there is no problem for the FC.
As said both methods work well, but the second one is more accurate when using arms with different length. I hope this “problem” with asymmetric frames, like mine or TBS Discovery, and the right settings is more clear now.