I have a 4 ring burner (380dia.) under a shrouded HLT 450dia. I find the 4th. ring is not economical to use as most of the heat seems to flow out side the shroud and just warm the garage. Three rings are good and do the job as quick as I can manage all the other preparatory tasks.
Get one of those nasa burners from globe importers they are great it has a better concentration of heat so u dont loose the heat around the sides and u need a hogher reg than normal and they heat up quick