Looking at a picture I saw on ********* I would say about 1 to 1.5 inches longer than the min. for prop clearance. So if you are using an 8 inch prop you would want the centerline of the prop to sit at least 5 inches off the ground. You may want to cant them back so your rear wheels are close as possible to the prop disk so you will avoid prop strikes, on rotation.
This is just what I would do, and to be honest, more guess than anything.