Nutrient levels

So i'm getting ready to set up my first hydro garden. But got afew questions. i have read several posts about proper amounts of nutrients they say to use a 20-10-10 or 30-15-15 mix for veg and a 10-20-20 or 15-30-30 mix for flowering. But all the nutrients i see to buy for hydro set up got listings like 2-7-6 , 0-.5-8 , 5-0-2. how the hell do i pull a 30-15-15 mix out of those numbers ? Could some one please help me out by telling me what you use ? like what type is it , what is the nutrients listing for it and how you would mix it for a 20 gallon reservoir ? Also i have read that you should water when the lights are on, is it safe to water when lights are off if ya think they are getting to dry ? Any info would be great. thank you folks and keep on token.

If your new and want easy simple nutes that comes with a light and feeding chart with exact ppm and ec listing just get fox farm grow big,tiger bloom and big bloom.Easy works great and is hard to over use these.The numbers your quoting are for soil.Hydro is much less as its direct feed and not filter by soil so little goes much further.Fox Farms will tell you how much of each bottle to add per gallon per week and what your light schedule should be and what your res ppm should be.Keep the ph at 5.8 and your good.You can fllod during dark but not as often maybe 4 times during lights on 1-2 lights off at most

I checked out that chart , good stuff there and read a little more on it. thanks much for the info .. wish me luck.