There are a few methods:
Method A: Thumb-suck. Otherwise known as trial and error. This generally has a success rate of around 50% and involves entering a value that seems reasonable then trying it out.
Method B: Looking at coverage plots. For ease of representation - and to help explain this - I've drawn three oversimplified coverage plots. In reality, a coverage plot will look something like this.
In the below plot, this is around -80dBm.
Method C: Drive testing. The MOTOTRBO Site Survey Tool allows you to a) connect a MOTOTRBO GPS model radio to a PC and record the coordinates and RSSI, in a file that can be plotted on a map or b) record the RSSI and GPS coordinates, of all radios transmitting on a specific repeater, via an IP connection. A real coverage map can therefore be created which gives the most accurate result.