Just use the 60 to 1 rule!

One degree is equal to approximately 100' per nautical mile (6026' feet).

So...

Your altitude in thousands of feet + (Radar elevation in degrees to top of storm x miles to storm)/10 = thunderstorm height.

Example:

At FL200 you have a thunderstorm at 40 miles showing the tops at 4 degrees radar elevation.

20 + (40x4)/10 = Tops at FL360

Make sense? You divide by ten to turn the hundreds of feet into thousands...

I know very little about wx radar so you may have to factor beam width etc... see above.