It looks like the way you hold it puts the two main buttons below the trackball, which I assume you operate with your thumb. If this is true, I have experience with a similar set-up, and I found moving my thumb from the pointer to the buttons to be awkward, as well as having the odd accidental click while navigating the pointer.
To my thinking, the ideal button set-up would have the left click button at the top of the device, where you would use your index finger. The right click should be to the side of the pointer - which might cause pointer drift, but would be far less awkward to press. The third button should be pressing in the pointer itself, or maybe adding a trigger mechanism underneath, which would be used by the middle finger.
Or maybe: top of device, used with index finger - left click; trigger underneath, used with middle finger - right click, see-saw button beside pointer, used with thumb - third button/middle click.
Still, JMO...
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