I use Jacks Classic in Coco Coir.
No PH issues (I don't adjust PH) No PPM issues that I know of (I don't read or measure PPM)
I water everyday unless I'm busy and forget then I just water them the next day. I do not water unless they are going to have at least 8 hours of light at the time of feeding, which means water them early or I don't water them at all.
I overfeed with Jacks I'm sure (Use the entire scoop it comes with in 3L) and I have no nute burn or any issues that I know of.
You can take a look at my plants, the last pic I have posted is about 1 week into flowering, yes 1 week.
It was recommended to me by various users on these forums that are "Pro" growers, no someone who does a few plants at a time, someone who grows huge massive plants and could be considered a commercial grower.
Obviously it will vary from person to person, but I have had nothing but a great experience with Jacks so far.
I've been told to use 20-20-20 through out the entire grow, although I have been giving it a half dose 10-30-20 (Of full scoop) for every other waters WITH a full scoop of 20-20-20 and I still have not burned them. It's working out great for me. I had a close friend tell me my plants look well into week 3 or 4 of flowering and I'm only 2 weeks in now (No current pictures in my journal since I'm just now finishing my new setup for next year and haven't had time to update)
So yes you can use 20-20-20 the entire grow. Save $5 and get the Duo as it comes with both 20-20-20 & 10-30-20 in one package. I do recommend trying the Citrus bucket as well if you have the cash, as I have also read great things about using that as a blossom booster. Although I won't be using that till next year when I really start to grow again.