First, let's review the two general types of servos:
1. Continuous rotation servos (also known as rate or speed servos) do not have potentiometers. This type of servo moves at a rate determined by user input and will stop moving when there is no input.
2. Position servos (also known as potentiometer or "pot" servos) do have potentiometers. This type of servo moves to a position determined by user input and will return to its original position when there is no input.
The general rule to follow when mounting the HoverflyGIMBAL: The HoverflyGIMBAL MUST move with any part of the mount that is controlled by a continuous rotation servo, but it MUST NOT move with any part of the mount that is controlled by a position servo.
Essentially, the HoverflyGIMBAL "sees" its own position relative to the craft, and thus it can work out the positions of each of the servos above it. However, it relies on the potentiometers to report the position of any servos below it. Thus, without potentiometers, it will not have the information it needs to determine where the camera is at any given point. It will still attempt to estimate this position, but without the necessary information, this estimate will often be extremely inaccurate.
If you have a continuous rotation servo on your pan axis, and position servos on your roll and tilt axes, then you would mount the HoverflyGIMBAL in a position between the pan axis point of travel and the roll axis point of travel (i.e. anywhere in the light blue area of the diagram). Note: This is the most common arrangement among commercially available 3-axis camera mounts.
If all of your servos are position servos, then you should mount the HoverflyGIMBAL above the pan axis point of travel (i.e., anywhere in the pink area of the diagram).
If all of your servos are continuous rotation servos, then you should mount the HoverflyGIMBAL below that point of travel (i.e. anywhere in the light green area in the diagram - essentially on the camera tray). Please note that this configuration is supported only in HoverflyGIMBAL firmware version 1.6 and higher.