Where that arrow points, the hollow range of the marking, are probably the most useful settings. I have mine just at the lowest point of the hollow marking. Whistler didn't make the knob with some stepped raster to keep it in its setting so it is easily knocked out of position. Or actually I turn it by accident as I usually forget that it's not its volume controller, that it logically should be. Squelch are usually a set and forget thing but Whistler have some squelch action issues so they probably gave the squelch adjust the highest priority.
/Ubbe