The M5 has dual beam 8/20 degree cone angle. If you use the 20 degree cone, to determine the bottom coverage area is to divide the depth your fishing in by 3. (30fow=10ft of bottom coverage area.)
The 8 degree cone (NBT is pressed) you divide the depth you are fishing in by 7. (30fow=4.25ft of bottom coverage).
This information is also in the manual. RTFM (Read the friendly manual)