A long time since I flew a 412, nor have any books available, but with the figures you give that would put the density altitude up near 10,800 feet. With an engine failure in those conditions I would expect the pilot may well be correct that he had a drift down rate of descent of 200ft/min. Nothing to do with rotor RPM, but not flying at the optimum speed (best rate of climb is 70 knots clean, 60 knots with main cargo doors open) could have an effect.