SOG yeild

I plan on building a SOG setup in my basment and am trying to get a clue to how much grass i will be growing? i plan on bending the tips over at around 18", I will be growing in soil and no c02 enrichment but i will have fresh air circulating and have a buch of computer fans to keep the air and heat moving. The room willl be ontrolled by a digi thermostat i bought i will hook up the ac on wires to fans that purge the system and the heat will be connected to a mini area hater. I will be using 2 500 watt hps? (what do you guys think about lighting to much or too little)

 

I will be growing plants each with around 13"x12" area for flowering and 8.5"x8" for veg, and a total of 6 plants per month (5 month rotation) so 30 plants total. Should i go shorter than 18"for a point to bend the tip? Will i get more yeild/time with shorter plant and flower it a few weeks earlier? how tall do most growers bend the tip over at i guess some of that has to come with trail and error