I use GH 3 parts too...for 8 week strain.
use aggressive veg mix - 1st 2 weeks of flower. That's right continue veg nutes for your first res in flower. You'll be ahead of the game if you don't have to learn that for yourself.
after 2 weeks change to trasition for weeks 2-4.
after 2 weeks change res to bloom for weeks 4-6.
at this res change I back off on nutes to 1/2 the flowering dose - as I only use this res for a few days.
This is where you can try out what you like best.. I have experimented with flushing and I'll tell you the truth, the runs I flushed were always better than the ones I didn't... but you can play with the amount of days you flush.. or not.
If you don't want to flush, I would still cut back on nutes for the last few weeks..
I have found that 10 days is my favorite for flush. So after 4 days I change the harvest res to water and clearex.
I usually go a week with that res out of conveinience but if I have time I do it at day 4.
When you run perpetual set ups, everything is run by the calendar.