I also use GH and coco (50/50 coco & perlite). I agree with the others about getting a TDS or EC meter. The "Aha" moment for me was when I started thinking about the feed mix and TDS separately. I personally mix up the mixture at about a 2:1 ratio (so 4 gallons of mix in 2 gallons of water), then I add more water until I get to the TDS I want. I follow the GH weekly 6-part medium feed chart almost exactly and I stay towards the bottom & middle TDS range for each week. This has worked well for me in my setup.
When my plants are seedlings I grow them in solo cups before I transplant them to 5 gallon AutoPots. While in solo cups, I water them daily using medium feed week 1 @ 250 TDS. With coco and perlite I've never had concerns about them drowning. It's just the opposite for me - I have to water/feed them daily because the liquid just pours right through.