If you want the most heat in the cockpit both levers should be pushed full in. That will direct the air flow thru the heater core and the water flow thru the heater core. My car is such a sweat box that even in the winter in Houston I do not run the heater. I always have the LH lever full in and the RH lever full out so I get cool air from the snorkel thru the fan and core but no hot water going into the core. If you also do not want any air flow into the cockpit pull the LH lever out and the door will cover the entrance to the core and cockpit and dump the air over board thru the slot on the bottom of the heater box. If you reach your fingers thru that slot and work the LH lever your can tell how the flapper door directs the air flow. My owners manual has instructions for the ventilation equipment on page 26, 27, 28. Hope this helps
David Barnes
68 E-Type FHC