If you are using the screen, run a normal photo period. You change the intensity by removing the layers of screen. You would remove a screen every week or so. Can't say for sure since I have never done it myself.
I have done the reduced photo time and you add time during the week but you don't use screen material.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,