checking ppm and adding nutes help

1)Im doing dwc, i was wondering how do i add nutes. Let say my water is at 300ppm and i want to get to 800ppm. am i adding 500ppm of nutes to the 300ppm water making it 800ppm or do i just add 800ppm of nutes to the 300ppm totaling at 1100 ppm?

2)how do i know when to add nutes during the week? Is it when the ppm drops? whats this add back rule thing about. Cant i just top off with water only since im changing out new nutes and water every 7days?

please help

im learning about the ppm's also and since im using GH nutrients i found this on their site, doesnt answer all your questions but might help abit...

WHEN THE RESERVOIR LEVEL BEGINS TO DROP, SHOULD I TOP IT OFF WITH FRESH WATER OR NUTRIENT SOLUTION?
Answer: If you drain, clean and remix the nutrients every 7 to 10 days, it's okay to top off with fresh water daily. As plants consume nutrients and water, the nutrient strength in the hydroponic reservoir will change. GENERALLY, nutrient strength should run between 800 to 1500 parts per million (ppm). Your exact ppm requirements will depend on your circumstances and style of growing. We strongly suggest that you measure ppm, you will need to purchase a nutrient testing device and start monitoring your nutrient solution. We use meters with a "sodium chloride" scale. Before using your meter, calibrate it with General Hydroponics Standard Reference Solution. Then, measure the ppm in the solution. If you find that it is greater than what you want, add water to bring the indicated ppm down to a safe level. If you find the ppm is too low, add nutrient solution to bring the indicated ppm up to its desired level. Your water quality will affect your final readings. When in doubt, remember that it is always better to apply too little nutrient than too much. Unless you know the specific ppm tolerance level for the plant you are growing, it is best to keep the nutrient solution between 800 and 1500 ppm.

BETWEEN CLEANINGS, HOW OFTEN SHOULD I CHECK THE RESERVOIR LEVEL?
Answer: Fast growing crops consume large amounts of nutrients and water so it is important to keep an eye on the reservoir every couple of days. Also consider the size of the plants(bigger plants, more water consumption) and how warm and dry the environment.(hotter, more transpiration).

DO I EVER NEED TO DRAIN THE RESERVOIR AND REFILL WITH FRESH WATER AND NUTRIENT SOLUTION?
Answer: Yes. The reservoir should be drained and rinsed every 7-14 days, depending upon plant size and nutrient usage. It should definitely be drained and cleaned whenever you change the nutrient ratio formula. It is also a good idea to rinse off the growing medium (Expanded clay pebbles, CocoTek coir) each time the reservoir is cleaned. Plant waste will tend to accumulate on the growing medium

try this wy m8[video=youtube;jsGty-DX0tQ]http://www.youtube.com/watch?v=jsGty-DX0tQ[/video]

Chee Zee nailed it. The reason for dumping and changing the res every 14 days is because the plants uptake different elements at different times. When they are younger they need much more nitrogen; later in life they need more phosphorous and potassium. If your meter says 300ppm, you still don't know the ratios; you might have 300ppm and no nitrogen, so that's the reason it's important to change; it resets everything back to where you are in control and not guessing.