Nutrient solution PPM question. HELP!

I'm starting my 4th week of my first grow and I need help with my nutrient solution. I've been reading online about how many PPMs to have your solution at for whatever week of growth. My problem is that I don't have a PPM meter and my spending limit has been reached so I will not have one for a while either. What is a good rule of thumb for adding nutrients? I've been adding about 5 gallons of water every week so far and I add in 5 gallons worth of nutrients. My res is 20 gallons and I have a 2X2 ebb & flow table with 6 plants. Is that enough nutes? Too much nutes? You guys with the PPM meters, how much nutrients do you add per week? I added just plain water this week because some of my leaves were starting to curl. I don't know if it was from too many nutrients of from the heat wave we had last week and my grow room got to 100 degrees...

I know that you said you "hit your spending limit" but I wanted to let you know that you can get a cheap PPM meter from about 15-20 dollars on ebay.
Without a PPM meter, I would use 1/4-1/2 of what is listed on bottle, and just watch how how plants are doing.