I was useing a 600w hps at two and a half feet the other day and still managed to burn my babies that was with a fan as well.. Are you controlling the heat, If so and doing it really well then 2 to 2/12 feet would be fine i think but please dont qoute me on that as ive only just started growing my self thats just what ive learned might be right might be wrong..
i use all 1kw so far so i would say at 11 days i was still at 5 ft by day 16 i was at less than four ft. now the plants a growing to the light never any burns but excellnt temp control.,