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.