Gday,
I used 300'/1degree C split quite sucessfully for years in Aus., particularly South and Central Australia.
But as Stally and others have mentioned, it may not be accurate in the more extreme environments.
Still a good rule of thumb though to give you a fair indication of as to when, if ever, you will pop out of the gloop if all you have is a METAR to go by.
Cheers,
Con
