What is the proper way in sentinel to program a custom channel using the green cross to edit? Do use need one of the custom numbers as long as you have the custom number enabled or do use the the service type as listed in RR.
You can choose whatever Service Type that you wish. As long as the given Service Type has been enabled in the scanner, the newly created channel will be scanned. (Of course Quick Keys, Avoids, etc. still apply).
You may have a good reason in your "scanning plan" to assign an "off" Service Type. For example, my local public works channels have been assigned to Custom 1, which I have enabled in the scanner. That way I can scan just my local public works channels, without hearing all other public works channels if I'm scanning the full database.