Just a couple of inches
You only need a couple of inches clearance. After you launch your boat, you can raise the forward and rear bunk supports equal amounts. Leave the center bunk support on each side slightly loose. When you reload your boat after having fun all season, adjust the center bunk supports, and tighten them. Making too high of clearance creates several problems.1. You need deeper water to launch / retrieve your boat.2. You raise the center of gravity of the loaded trailer, making it less safe to tow.3. You have to lower the keel more when trailering, allowing it to move laterally and cause wear and damage to the hanger assembly.4. You'll need a longer ladder to get onto your trailered boat.