As far as i know, most people use the '3000 lumens per sq/ft' technique. the sun provides somewhere around 10k lumens per sq/ft in nature, and some would argue that you should aim for something closer to that. really depends on who you talk to.
as far as i have seen, 3k will get you by fine, especially for your first grow (if its your first, with 3k, lights wont be your issue, youll be more worried about turning your thumb green and learning the process).
of course, dont forget, if you can KEEP HEAT UNDER CONTROL, there is no limit to how many lumens you should aim for, the more the better, always.