Don't flush unless you notice problems with your plants.
You are growing in coco/perlite as I do, but use different nutrients. Just for the halibut, I checked the run off from a Cream & Cheese. It's at pH 4.43. The plant is 16 days into flower, and the nutrients going in have been at pH 6.2.
OK, downstairs I checked the other plant. It is 34 days into flower, and is getting the final ripening formulation at pH 6.2. The run off is at pH 6.52.
There is a noticeable difference in color of the run off between the two plants. The first has a very dark run off. It gets 1.5l three times per day, and there is very little run off. The second plant only gets 1/2 liter twice per day, and has a run off of close to 15%.
If you are not getting at least 10% run off, I suggest giving your plants more. I'll be doing the same for the first plant I checked.