All strains are different so you sort of just need to do what your doing...slowly raising it till you get a little burn. For instance, right now my G13 and TW are just loving 1600ppm, but they are around 3 feet tall and 5 weeks into flowering. My Alaskan loves 1600ppm and its much much smaller, it just eats a ton. However my Cindy 99 and Afghani Kush can only handle about 1200 right now.

I start at 275ppm, horrible water here. I use 1 part GH Flora Nova. I am assuming you have a PPM meter? Your mixture can be much different then your labels may suggest. Plus you never know with your water (unless you are using RO of course).

When my plants were about 4 weeks old, I was giving them about 600ppm of nutes giving me about a 900 total ppm. I use the same res for a few different strains so I have to sort of find an average between them.