For my nitro 1/10 scale sedan: 4-5mm on front and 5-6mm on the rear. Depending on how new and smooth the surface is, i'll go lower sometimes.
For my nitro 1/8th scale on-road: 5-7mm up front and 7-8 mm on the rear. Again, if the surface is new and smooth, i'll drop her down lower if needed.
Just keep in mind regarless of what right height your trying to accomplish for on-road nitro applications, you'll bottom out every so often. I haven't met a racer that didn't have any scratches on the bottom side of his chassis if racing on asphalt or concrete. Goodluck.