When you draw your two boxes ( I usually use one around the item being imaged or a corner star if you can get a fast exposure rate and still see the star and then the second around a convenient star in the opposite or far corner ) it will de-rotate as it stacks if it needs to. then pressing guide here will try and keep the first box item in the same position.
Centre target will try and centre the first box item in the fov, but I don't find it very effective, and can't really see the point.
Set target will let you draw a single box around an item and attempt to keep that in the same spot once you press track here.
FL in mm is more for the fits header I think, I've changed it and it makes no obvious difference on my setup.
Corr Gain I have only played with a bit, with 2 cam's it is a little redundant, because you set the tracking / guide cam as low exposure ( quick screen refreshes ) but if you are using a longer exposure, you can increase the corr gain to help it get closer to the target it is trying to centre ( does that make sense ?) ( ie. it will move the scope a little more depending on the figure in corr gain to try and reach the target spot )
So basically, the image coming in on the device used for tracking you want as quick an exposure as possible so it can adjust your tracking as accurately as possible. So a bright star is better, but not essential. The image of the guide star doesn't have to be great, just good enough to always see the star and it's shape while keeping the exposure time as low as possible. My understanding is that as the image is processed in envisage and the change between the track here target and it's latest screen refresh determines how much and in which direction it moves the scope.
Draw a couple of boxes at opposing sides and press track here. Job Done !
One last thing to consider is the orientation of your camera in the scope. I mounted my guide cam in the wrong orientation, and as envisage tried to track, it was moving the scope the wrong way .. took me a while to work that one out, but I'm getting old
