could someone explain the light cycle to me please

i start cuttings on 24 hrs light, keep them on that until two weeks after they go into the main tank, then 18 hrs until their about two foot tall, then down to 12. I think the old rules around flower at 2 foot if you want a 4ft plant don't generally apply to sativas. i normally only grow sativas and under a 600w hps i put them down to 12hrs when their about a foot tall. i raise the light to stretch them slightly as i find i get more bud sites. all the sativas i've grown over the last 15yrs have ended up at about five to six foot high when doing this. the only exceptions are when i've grown from clones. found these need to be in 18hr light a few weeks longer or you end up with bonsai plants that rarely exceed three foot. my last plant that went this way stopped growing at about 24inches, and gave me about 1.5oz of dried smoke after about 6wks in flowering. nice if your desperate for a smoke, but depressing in comparison to the huge grop you generally get from something like northern light or orange bud if left to grow to a good size. psteve is spot on, save arsing with lots of different lights, go for the 600hps and be done with it.