A cheap, easy, no-fail method is to use window screening, mentioned above. Go to Home Depot or some other local hardware store and buy enough window screening to make about 5 sheets as long/wide as your tank. Cut out the 5 sheets and lay them over your tank (you'll need to tape each one down with duct tape). Then, keep your new lighting at your current photoperiod and remove one screen every other day.
This way, you don't have to worry about setting your timers every day, etc., and screen is cheap.
By the way, what type of lighting was replaced?