YSB,
Most modern radars generally have 3 deg beams and as you know, using 1 in 60, 1 deg down at 10 nm = 1000' at 20nm = 2000' etc
To keep it simple I use this technique. I set the scanner tilt to -0.5 deg. so the bottom of the beam will be 2 deg down. 2 deg down at 10nm = 2000' and so all you have to do is double the range at which the return just appears to get its height difference in 100s of feet. So it just appears at 80nm it is 16,000 low.
For ludites like Buzzy, if you want to check out the radar's calibration. Say you are at 40,000' and put the antenna 10 deg down (-8.5 tilt) you should see the ground return at 40nm.
Ghost