It depends. Think of -10 like 55, not something automatic. Lower gain could reduce range, but not necessarily, depends on the noise.
Low, close flying airplanes or with very high gain high flying, maybe 20-30 nmi distant airplanes might have a signal that is too strong to detect, so you basically lose some planes at some point.
So you could for example reduce gain until you start losing range and then go back up a bit 
Or you could check for strong messages with the command mentioned here (Thoughts on optimizing gain - #2 by wiedehopf)
Anyway read my two posts in that thread for some explanation. Like the filters, some trial and error is involved that can’t replace understanding.