Variables. There are more variables. The Stinkster is right...so either you have 1200w but have that light way up high to chill the PPFD down to a nice reasonable number, or you need to dim and can leave the light down a bit. DLI is light exposure across a day, so it's calculated by figuring out your plant's optimal PPFD in whatever stage (seedling), and the amount of time during the day it's exposed to that light. So a 300PPFD may be just right at 18/6 but will be too much light at 20/4 or 24.