Sorry for the late reply but life happened. Yes I recommend a flush at the end of the 7th week. Here is a link to a post in another thread about the feeding schedule:
FF Schedule
They have their own flush formula but you can also use plain water.
What you are trying to do is remove as many micro particles of nute residue as possible. This unbinds the nutes from the roots and removes them helping the growth. When you flush at the 7th week it is close to the plant flowering and you want a fresh start when you start feed her a different formula for the flowering stage. I use 3X the amount of water that I usually use.
