Bloggs.............
The LCL calculation starts to give a lower cloud base than the rule of thumb method as surface pressure reduces. It also does as dewpoint spread increases. So if you've got a low pressure and a high dewpoint spread then 'reliability' of the rule of thumb method reduces. Not by much, but it confirms what Stally says.
Anyway, head on down to Victoria RFN, because you'll have the perfect opportunity to start trying out both methods with all the low cloud that BOM's arranged there at the moment
