A bed? I'm not sure what you're asking. You only need to have the grain bed deep enough to "filter" the wort as it runs out. I don't use rice hulls, and I've never had a stuck sparge, but I've never used more than 50% wheat either. If you're using rice hulls, a half-pound to one pound will be enough to help with the sparging.
Broken Leg Brewery
Giving beer a leg to stand on since 2006