To get the height of the guides right - hook the trailer to the truck -
back it down the ramp you will use most of the time -
can you see the tops of the guides when the trailer starts down the ramp?
Yes = tall enough - No = Nope, not tall enough.
One other thing - when you get it all done, take the boat to the ramp and back it down.
Just when the boat starts to float, get out of the truck and make a mark on the driver's side guide at the water level.
When you get it home and dried off, paint that mark so the next time you are backing the boat down the ramp you will know when the trailer is far enough back to launch or retrieve the boat.