Another method that works for me and is simple(ish)
Take your ht AGL in FL's and divide by 10 (eg FL300 over water = 30)
Paint the ground at this distance
Note the tilt (say -4deg)
Add 10deg (now +6deg)
The bottom of your beam is now parallell with the aircraft at FL300
Start tilting down untll you get a return
Say you pick up a return at 50nm at +3deg, that return is 15,000ft below you (50(x100) x 3 deg.
As previously stated you may have ice crystals well above this level which are not painting.
You don't need to know your beam width for this tech as you are using the bottom of the beam. Your beamwidth however will vary with the size of your radar dish (the bigger the dish the smaller the beamwidth)