I think there are a few factors that will make fish prefer certain depth on certain days: atmospheric pressure, light conditions, water clarity, water temperature, tide, water surface (wind), presence of bait fish etc..
Fish at the bottom feel safer so would be biting more. There is less visibility down there so they know it's harder for a predator to see them and they also see your fishing line and hook less well.
Also instead of changing leader length and waste time undoing and redoing knots I would stick to a 1m to 2m leader depending on the depth of the area and use an adjustable float stopper above the float. So the rig from top (rod tip) to bottom (hook) would be: float stopper, float, float stopper, swivel, leader with split shot and then hook.
Note the stopper below the float this allows you to set minimum length between the float and swivel to help casting and prevent tangles.
Depending on the current you might want to add a sinker above the swivel or use more/bigger splitshot above the hook.
The split shot above the hook should be halfway on the leader or 1/3 (closer to swivel). It allows straightening the leader a bit and also helps with tangle by keeping the hook away from swivel and float when casting.
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