To change the threshold on a system you can go into Sentinel and highlight one of your favorites lists and then click Edit and Edit Favorites click OK. Once the favorite’s list editor comes up off to the far right you should see an option that shows threshold mode (which you can change from Auto to Manual) and then you will see the threshold level (which you can then change to from 0 - 20). Then click file save and then file exit.
You can also do it on the fly in the scanner menu:
Press Menu and scroll to Manage Favorites and press the volume knob and scroll to the list you want to change and press the volume knob to select. Select Review/Edit System then scroll to system and select. Then select Edit Site and scroll to P25 Threshold Mode and change it to Manual. Then scroll to P25 Threshold Level to select the level you want. Then keep pressing the menu button save and exit all the way back out until you start scanning.
Unfortunately I don't have the in-depth technical knowledge to explain exactly what is happening, other than it has something to do with levels needed to cause the scanner to react to the incoming signal.
There is no "universal" number. Each radio system and user's location has certain variables affecting the signal. So we are all in the "trial and error" mode when it comes to determining which values work best for the given system we want to monitor.
See the screen shot below on where these adjustments are found in Sentinel, when editing a Favorites List: