I didn't want to raise the outer bunks any higher than I had to. I didn't want to hit the crossmembers if I wasn't deep enough or whatever. I made short aux. bunks that cushion and center without a roller. I had a roller on the last boat and I like this better. All my bunks get sprayed with liquid rollers. Your "how deep" question really depends on the ramp steepness also so kind of hard to answer. The rear top of my fenders are above the water at the normal ramp.
