I'm not sure where a "number value" factors in for the 1088. The 1088 has a squelch knob. Typically, I have mine set to about "10 o'clock". There is a marking around the knob and on the left side you'll see a clear space that indicates the suggested range of setting. Of course, you have the option of setting it to what ever you want. Too far counter-clockwise can slow the scan rate down (to a stop if you have conventional analog channels programmed). Too far clockwise and you may not receive anything.
I believe that the only Whistler models that this would apply to would be the WS-1095/1098 and TRX-2. The 1080/1088 and I'm pretty sure the TRX-1 has an old-fashioned potentiometer-controlled squelch that's only adjustable via the knob, and it doesn't use level numbers. Set it at about 10:00 and if you still aren't hearing something, gradually turn counter-clockwise until the radio hisses, then turn it clockwise until the hissing just stops.