You could use the 1 in 60 rule:
ht above a/c (ft) = range (ft) x (tilt - half beamwidth) / 60
which you could change a bit to be
ht above a/c (ft) = range (nm) x (tilt - half beamwidth) x 100
assuming 6000 ft = 1 nm.
This all assumes of course that the bottom of the Wx radar beam is just touching the top of the cloud and you can just see it on the screen. If the (tilt - half beamwidth) bit is negative the left hand side becomes negative and the tops are below the a/c.