I’m sorry, but whatever way you put it, your idea is “reversed”. How can you measure the rxlev at the horizon ? And why 6dB ?
1. For each antenna pattern, in order to achieve 6dB attenuation at horizon, a certain tilt is required. Not all antennas need the same tilt to achieve this…
2. The most effective tilt is found by computing the distances (geometrically) between :
the BTS location and the 0dB point
the BTS location and the -3dB point (upper)
3. When you cover large areas, you certainly want your horizon to be covered as much as possible.