I take it your using synthetics.....I would feed till the last few days (by looking at your plants trics and going Mmm 3-4 days) and simply straight pH'ed water till done......There is plenty of nutrients in the soil to last that short period..
In my experience, I have almost never had any strain do an actual "8 weeks" etc. as a breeder report says. The best quality is done by watching and learning to "see" when the plant is done and harvest then. Usually, 1-2 weeks longer then reported, sometimes a bit longer......PATIENTS is the key to growing - harvesting and drying/curing properly