HY
Check this link
http://www.avweb.com/news/airman/184282-1.html
From usual practice I can say that ,at cruise,we keep tilt on 0. Everything that still appears on the screen ,bellow 40NM will be at ,or very near to your level.
Sometimes the eye can deceive you.Especially at night ,over lighting storms .Even on daytime you can easily be wrong about the height ,or intensity of a cb.I often see clouds that are not painted on screen,although they are quite big.But you must consider the radar 'sees' water,so if there is not such a big amount of water,(sometimes in a TCU) it won't appear on the screen so big as you see it.
It's better to trust your radar.
Hope this helps
Alex