I don't do this as a matter of course, it's something i mentioned as an easy and effective way to help increase the 'richness' of your water borne coral nutrition. It keeps a constant supply in the water rather than throwing some bottled BS in once a day etc.
I always skim wettish - dark tea color like most guys. There's no need to adjust anything and over think things, just let whatever you normally remove cycle back through the system and watch your reef to see how things react.
If you see positive signs from doing what i suggest - stop the skimmer draining thing and start adding a food that will quickly increase your nutrient levels such as mysis.
The only way you can not get a reading from your water is by not feeding your bio filtration the amount of food it can handle. I feed 5 mysis cubes a day, nori and multiple pellet feeds and my nutrients are still dropping - i will feed more until i reach a balance because the more food i can process daily the better my acros look.
Excellent advice. Tomorrow I am having a fitting and hose modded onto my skimmer cup.