i think both grams per watt and grams per square meter are just guidelines to refer to a certain amount of harvest based on setup.
most people don't calculate anything but the light itself in the gram per watt measurement. so if me and you both use a 400w hps and each get exactly 400 grams, but you used more fans and filters than me, i did better than you, even though we both hit the gram per watt mark as far as the light is concerned.
as far as the gram per square meter measurement, i don't even fuss with that one. i run a semi-perpetual setup, and my flower room isn't always as full as it could be, so i obviously wouldn't hit whatever the mark is supposed to be.
the way i measure an indoor harvest is to look at the cost:return ratio. how much did i spend to grow it compared to how much i would have spent to buy it on the street? if i spent a hundred bucks a month over a 3 month period, how much more herb do i end up with than if i spent the 300 on a sack?
my personal goal is to spend no more than $1.50/gram to produce it. this is a bit harder with a smaller light and fewer plants but i usually shatter that goal so i don't much give a shit how much i yielded as long as i have enough stash until the next harvest. if i was growing commercially for a profit my standards would be a lot higher.