Good questions Smitty. Part of my daily routine is to check the water level, PPM level, and PH. You are right in that evaporation can exceed nutrient uptake and inversely affect the PPM levels. For the entire vegetative stage and for about 3 weeks into the flowering stage, the plants were uptaking almost equal parts of nutrients and water. Therefore, I would refill with full nutrient solution and the PPM levels would remain the same. However, over the last few weeks, the water intake has been much greater than the nutrient uptake. So each time I check the PPMs, after the reservoir level has gone down, the PPMs ratio has gone up. I then only refill with PH adjusted R/O water until I obtain my desired PPM levels.
Every other week I fully drain each reservoir and refill with pure R/O water with Clearex solution, which I run for 1 hour. The Clearex leaches out the salt deposits in the reservoir, roots, and the plant itself. I then drain the Clearex water and replace with new, fresh nutrient solution.
Also, I believe that over the course of two weeks, the plants take up certain needed elements in the nutrient solution while leaving others behind. Therefore, the balance of overall elements gets out of whack. By starting fresh every 2 weeks, I know I'm getting a balanced supply of elements in the new nutrient mix.
Clear as mud??