Ok, first of all about the nutes, you are probably still a little young for them right now. If they start to get a little jaundiced (yellow) overall then is a good time to start the nutes. Later is better than sooner.

Without a TDS meter you cannot do any calculations. First of all because you don't know what your base of the water is. Is it distilled water with 0 ppm? Is it tap water with 300 ppm? Did you add CalMg?

So while you don't have a meter, do this: Get distilled water. Add a tsp per gallon of CalMg. Then do 1/4 strength of 600 ppm for your nutes. If for instance the Sensi label says add xxx tsp/tbsp per gallon for 600 ppm, do one quarter of that.

Now watch for a few days and see if you get super yellowed tips. That is a sign of too strong nutes. If that happens empty half your rez and top it off with more distilled water.

Once you get your meter, come back for more advice. The next step will take a meter.

Luck man.