Calibration

The gimbal will work reasonably well without any calibration, but for optimal performance the accelerometers within the IMU modules need to be calibrated (the gyros too, but that's currently beyond reach). Several calibration techniques are existing, and two are implemented in the STorM32 project, these are the "1-point" and "6-point" calibrations.

Unfortunately, it is not absolutely clear which method is the best. The firmware author doesn't have the resources for an extensive research project, which is what would be needed for a determination. Hence, the situation is a bit experimental, but - on the positive side - knowledge and experience is increasing and therefore calibration procedures may improve with time.

General

The reasons for a calibrating are different for the camera IMU and the 2nd IMU.

Camera IMU: Calibration mainly affects "horizontal drift", i.e. the accuracy with which the horizon is held in quick yaw turns and other high-g maneuvers. It also determines the accuracy of how well the camera is leveled, but this could in principle be tuned out easily by other means (e.g. offset parameters). The main concern is the performance in high-g maneuvers.

Second IMU: The measurement accuracy of the 2nd IMU is by itself not very important. It is however very important that the 2nd IMU data is consistent with the camera IMU, and it is even more important with higher poled motors e.g. poles greater than the basic 14 poles.

It has been found that the calibration requirements are quite different for the different IMU axes.

IMU Orientation: In general, the Z axis is more often wrong than the X and Y axes.

Of course, the care with which a calibration is performed very much affects the final quality results of the calibration.

Calibration Accuracy: A 6-point calibration is meaningful only if it is done very accurately. A poorly undertaken 6-point calibration can easily result in poorer performance than a supposedly "lesser" 1-point calibration. The resulting quality factor for the 6-point calibration option can give a good indication as to the results (a value of 80 is very good, 150 average, 250 not so good).

From the above information these are rules of thumb to follow:

Rules of thumb:

The calibration only needs to be as good as you need it to be. If you are happy with the way your gimbal performs, you don't need to spend time on a (better) calibration.
IMU orientations with the Z axis pointing up or down seem preferable, as it lessens the need for calibration. But don't be fooled here, equally good results can be obtained for other orientation. Some of the best videos out there were in fact recorded with the IMU's Z axis in a horizontal orientation. It just means that with the Z axis pointing up or down it's more likely that one can get away with a "lesser" calibration for a desired level of performance.
The calibration of the camera IMU is more important than for the 2nd IMU, i.e. for the 2nd IMU a "lesser" calibration might be fully sufficient.
The calibration of the 2nd IMU becomes more important as the motor pole count increases (e.g. 14 pole versus 22 pole).

Even though, for the reasons mentioned previously, a clear-cut calibration recipe cannot currently be offered, there are some recommendations in place:

Current Recommendations:

A 1-point calibration is fairly simple, and is hence recommended for both the camera and the 2nd IMU.
If possible choose IMU orientations with the Z axis pointing up or down, but that's not a must as explained before.
Don't do a 6-point calibration unless you plan to do it very accurately (check out the rcgroups thread).
If motors with 12 or 14 poles are used, a 1-point calibration of the 2nd IMU is most likely sufficient. In contrast higher-pole motors require a careful 2nd IMU calibration.

Finally, some important advice:

Save your calibration data to a file!

Ideally save it from within the [GUI:Acc Calibration] dialog window, as this stores the most detailed information. Consider also posting your calibration files in the rcgroups thread, so that the firmware author and others can enhance their knowledge and hopefully develop better calibration schemes.

Calibration Procedure

The calibration consists of two steps. In one step, the calibration constants are determined for the particular IMU module under consideration. In another step, the calibration constants are made effective by storing them permanently into the STorM32 board.

The [GUI:Calibrate Acc] tab provides the means to select the IMU module, to enter calibration values, and to store them. The fields which hold the calibration constants can be populated in several ways:

Values can be edited or entered directly into the text fields.
The values are set to the default values by hitting the [Reset to Default] button.
The values currently stored in the STorM32 board can be read out by hitting the [Read from Board] button.
The fields can be populated with calibration values stored in a calibration file via the [Load from File] button.
The fields can be set to the values obtained from a 1-point or 6-point calibration via the [Run 1-Point Calibration] and [Run 6-Point Calibration] buttons.