How do you top off your nutrients?

[jalayo]

I'm new to hydroponics, and I have spent the entire evening, with no luck, searching the web for an answer to my question. I have built my own flood and drain system for tomatoes and peppers. I have a 45 gal reservoir and four 5 gal buckets and four 3.5 gal buckets. The plants are growing great and looking very healthy. I have run my nutrients for about three weeks. The lady at the local hydro store says to change them every 2 weeks. (But she is also the lady who sells the stuff). I have also seen that same comment (2 weeks) on this forum as well.

Today I got an ec tester and the reading for my nutrients was 1450. It should be higher for tomatoes. My question is how do I know how to mix my nutrients, so I can top off my system, so I won't burn/kill my plants? Is there any formulas that would help me calculate how I should mix my nutrients so I can top off my system and get my ec up to about 2400?

Thanks in advance.