Can’t tell if it needs to be flatter or not. I would have to see a ‘scum line’ on the hull of the boat or see it floating in the water to know for sure. You can drive by a pole or bridge piling and if it appears on the Si sonar to be leaning towards the top or bottom of the display than you would get prettier Si sonar images by leveling it out some.
As for far low enough: I would just try it as is and make adjustments as needed.