I hate to sound dense on this, but he used your exact formula from above. Could you look at your original calc post and his again?

Even still, .5 gram per month or 1 gram per watt per harvest aren't just good growers, they are fantastic growers.

Quote Originally Posted by stinkyattic
In line 3 of his post, he uses both '0.5' and '60 days'.
0.5 is the multiplier for 'per 30 days'.
Given twice the time, your multiplier becomes '1.0'.
Read post #6...

Under a 400w lamp, you should get 200 grams per month of flower time.
If you run flowering 2 months, that makes 400 grams.
If you run flowering 1.5 months, that makes 300 grams.
Those are your target numbers with that light.