The recommendation is if you are overflying a CB to be at least 5000' above it , difficult with tops around FL400.
Weather showing on your radar 80 miles ahead which looks acceptable can be very different when you arrive there 10-15 minutes later. During that time cells can easily build up and returns get stronger as you get closer.
A towering CU doesn't show up very well due to its low water content but the turbulence can be quite severe so use mark 1 eyeball in addition to the radar