Exactly, not that hard to understandOK.... If you get 1.25 GPW... And your plants veg for 16 weeks...
and Bloom for 14 weeks... you did good at 1.25 for your 30 week grow....1200 watts so you got in 30 weeks = 1500 grams....
During that time, [I get .6 grams per watt]
I did 8 week veg 7 week bud... two complete harvest for your one... so my .6 = 1.2 over your time = 1440 grams for my little .6 per watt grow...
It's all good...