Light?

DLI is what you need to focus on. Same DLI is fine... more or less might be a problem. Depends on hours of operation which wasn't given. If it's an auto on 18h, 600ppfd is all you need, assuming it is accurately calculated. Over 12h, 900PPFD is the same DLI. This is no coincidence. Regardless, you need to observe and adjust further from any suggested starting point. Better to get this dialed in while vege growth remeains -- node spacing of new growth is an excellent guide. If you figured it out for vege, then it's easy math for flower. 1:1 inversely proportional to change of hours of operation. 12 from 18 is 2/3rds, so 3/2 power for same DLI. 600 x 3/2 = 900. Easy peasy. Assumes all other relevant factors remain the same... e.g. hanging distance, temp, rh, co2 etc...