[quote]As previously stated you may have ice crystals well above this level which are not painting.
Agree. Read the POH for the type of radar you have. Rockwell Collins recommend use of MAX gain in cruise at high altitude to pick up the slightest echo which indicates a ice crystal top (may be well above you too) and then a combination of gain and tilt to gauge the size of the cloud. Works a treat in the Pacific region - don't know about over deserts. Either way, it is not worth the trouble of maths calculations to find cloud tops simply because of lack of reflectivity of anvils and CB tops. Old fashioned rat cunning with use of gain control is the best way to fly safe.