I may be able to help. Although many would disagree I keep my nute strength (GH 3 part) at 200ppm for the first 2 weeks. I change solution every week religiously. I maintain 5.8ph and do not use RO water.
As a side note....I highly suggest that if you're using tap water you let it sit for 2-3 days BEFORE correcting ph and adding nutes. Originally I'd fill a resevoir (For me, water straight out of the tap is about 7.0.), add the nutes, adjust the PH and fill the containers. Then I'd spend the next 2 days adjusting the PH thinking that the rising PH had to do with the plants. Not so.I guess it had to stabilize. NOW, as I said...I let it sit for 3 days after getting it out of the tap and before draining/filling. THEN I add nutes, adjust the PH and change the solution. This is what they look like 8 days after sprouting.
Lastly...Solution strength is dependant on several things.NOBODY can tell you the 800 or 1000 or 1500 is a good number. Start low and you won't hurt them. Start high and you're in trouble. Here's an easy way to think of it. If you measure ppm today and it's 600, then you measure it tomorrow and it's 500, your plants are using more nutes than water and you can go a LITTLE higher on the next mix. If the PPM is 600 today and 700 tomorrow the plants are using more water than nutes and you should dilute the solution.
The pic below is 8 days after sprouting under 800w of MH and 200ppm at 5.8.
Good luck!