Hi Everybody....I have grown things hydro phonically , many years ago, and now ,am starting again with tomatoes (indoors under LEDs).

From my previous experience , I got the impression that people seem to make the nutrient thing very complicated and expensive, when it should be neither.

I'm starting with a bag of 15;15;15 with trace elements, instant release. This bag I bought for $30 (In Thailand) that's 60 cents/Kg... I plan to use only this.
Now I know this is not the perfect balance, but I'm not growing commercially, only a few square meters, so I plan to adjust the strength to make sure they get enough nitrogen ; they will have too much P and K which will tend to build up in the solution, but If I change the solution regularly this shouldn't be a problem.... should it?

So my question is , how many grams/liter of this fertilizer do I need to put in rainwater to make it fine. for (a).seedlings and (b).full size plants?