This trim tab helps compensates for the motors torque by turning the motor and it's a bit confusing at first as to which way to move it.
If you turn it towards the boats right it will push the motor to the left, which helps turn the boat to the left (reducess the pull to the right) and visa versa.
Ideally, you want the steering to be neutral - not pulling in either direction - when you're at cruising speed, which will leave you with some pull to one side during take-off.
If you change your prop you may have to re-adjust the trim tab. The steeper the pitch or deeper the cup on a prop the more torque it will produce.
You can affect the boats attitude in the water somewhat by adjusting this trim tab but if you're trying to make the boat stay level in the water from left to right when you have a greater load on one side you will have a lot of pull on the stearing wheel while cruising (unless you have power-steering).
If you boat by yourself alont try to move as much weight to the left side of the boat as possible to help balance the load. The boat will probably still list to the drivers side but you'll get used to it.