ok, here's some information for you.

You can start nutes as soon as you put the seeds in rockwool. I tested this and survived up to 500ppm to start with, i use 200 for saftey.

Im going to say start at 400ppm in your res, test how much of your nutes adds how much ppm to that 20 gals, and even them out.

I figure some advice is better than none. i have not successfully completed a hydro grow, but these ppms have not killed my plants with fox farm nutes.

those ml's to the gallon cant be answered, you need to check your nute levels via PPM, not how much you add.