You can use 24/0 light or 18/6 to begin with veg phase...the differences between the 2 are debatable...I use 24/0. The 12/12 is for the flowering phase, although 13/11 seems to be used lately with good results(bigger buds?). It really makes no difference, I have noticed, how you split the additional hours of light when adding hours of light...at the start, finish or splitting the difference. This is usually a one time thing and the plants will adjust to whatever you give them after a few days regardless of how you divide it up. Depending on what type of lights you have and how much electricity you want to pay for should be a consideration as 24/0 will cost you another 25% over 18/6. I actually did 12/12 to induce flowering for 2 weeks and then went to 13/11 (where I currently am at this point in this grow) We shall see what happens! Good luck and I hope this helps!