I used multiple layers of "screen door" screen I started 8 thick and took a layer off every 5 days slowly increasing the intensity of the light entering the tank.... worked for me going from 260W PC (130W actinic/10K) to 760W MH and PC (500W DE MH at 10K and 260W actinic PC)
JME other have decreased their photo period with MH
there was a thread just recently
http://www.canreef.com/phpBB2/viewtopic.php?t=17644
There it is
HTH