Very difficult to give a hard & fast figure as I've found different vans give different readings due to different constructions. With the DT128 I find anything below 20 is generaly OK but the best way is to check lots of different areas till you get an Idea for that van. Also don't panic if you suddenly get a high reading, remember you are basically measuring the conductivity of the material so a hidden cable or metal bracket etc. could be the cause. If you do find damp the reading should get get progressively higher or lower as you get nearer or further away from the source. It takes practice but if you go over your van regularly you'll get to know its "normal" readings..