When you take an EC or TDS reading after the plants have uses up some nutrients they also put back others it will read different. It does not mean you can use fewer nutrients. That's why you change the water/ nutrient solution every 7 to 10 days. by day 15 the water/nutrient solution can become toxic to the plants. So it's better the error on the side of caution and play it safe.

You may need to bend the tops over and tie them down to make room. On average most strains grow about 12" to 18" during the flower stage.