This is a copy and paste from the latest Rockwell Collins WXR-2100 Multiscan Radar manual equipped in my fleet which begs you to leave it in fully auto gain.
"The top of a thunderstorm is composed entirely of ice crystals (glaciated) and reflects very little radar energy. At temperatures less than –40 oC, liquid water no longer exists and only ice crystals are present. The altitude at which this temperature occurs varies depending on the time of day, time of year and based on latitude and longitude. Ice crystals are formed when the positive hydrogen atoms and the negative oxygen atom of water molecules lock or freeze the molecules into an ice crystal lattice. Ice crystal molecules are not able to change orientation to respond to and reflect radar energy. The middle portion of a thunderstorm occurs above the freezing level (0o C) and up to the altitude where the outside air temperature drops below –40oC. This section of the storm is composed of a combination of ice crystals and super cooled water. The super cooled water provides moderate reflectivity, but some reflective energy will be lost due to the presence of ice crystals. The top of this section of the storm is often referred to as the wet top or radar top of the thunderstorm. Building thunderstorms also have a turbulence bow wave that extends well above the visible top of the thunderstorm. The bow wave may cause severe turbulence but is completely invisible to radar."
Having read that, I found myself avoiding cells that are sometimes 4 to 5000 feet below me or cells that are not even there...so guess what? ...2 clicks out of auto gain and you really have the correct picture on the nav display, but I leave the F/Os settings on auto just in case the bow wave is too obvious...