OP you don't wanna cut down on your light period!
since most cannabis will flower with 12 hrs of darkness you don't want to give it any more of a dark period than that! why? you want them to get as much light as possible while still being able to flower...this way the plant will grow to its maximum potential.
Its ok to use less than 12hrs light if suggested by the breeder (some strains love less than 12hrs of light in the last two wks...like most white widow varieties).
i personally grow like this: 18/6 for veg...24-48hrs darkness...then flower at 12/12. then i give 48-72 hrs darkness before the chop.
i put them in the dark for 1-2 days from veg to flower to give them a kick start. ive noticed that i actually see hairs a few days quicker with that method versus just going from 18/6 to 12/12
i put them in the dark for 2-3 days right before harvest to milk them for every bit of resin/thc goodness they can produce
im growing out a white widow right now and they suggest to give it only 8 hrs of light in the last wk or two...this will be the first time i give less than 12 hrs of light for flowering but only because the breeder suggests doing that..
other than that just stick was 12/12 and give them the best chance to give you a good harvest.