FYI I’ve found that my 23 is fairly stern heavy due to the weight of the motor and other furniture/accessories aft of the center line. As a result #1 feels way too low and I can even trim up past 3/4 before any sign of ventilating. I’m moving it to #3 next week and if successful I may just mount a hydraulic jack plate with a short setback so I can adjust based on conditions.
Hit 49mph at 5700 rpm, my prediction is 51-52 ish at 6100 after motor raised.
Point of all this is with the 300 you may also squat and could benefit from moving the motor up. But as far as where to start - you’re going to move it again regardless so probably doesn’t matter (unless you start at 4 and ends up perfect).