When milling wide boards, I often put a line in the middle of the board so I have something to target. This helps me weight my plane properly so that I actually flatten the board.
To quote Robert Wearing in his book “The Essential Woodworker” – “At the start of the stroke there is strong pressure by the left hand on the knob, while the right hand on the handle pushes straight forward. In the middle of the stroke the down pressure is equal by both hands. At the end of the stroke the positions are reversed … “.
The way it was explained to me in my first hand planing class is that you’re digging out a bowl. So my red line is the middle of the bowl and I work to make it disappear.