Confused about PPM and pre-amended soil and knowing which ppm to sart feeding.

Just noticed your pots are 5gal so 1L wont be enough. My pots are 3 gal and I use 2L of water each feed and use a total of 4L when I do my mini flush. You'll probably need 3.5L each (3.5L water & 3.5L nute mix) With this method you need to give the plants time to dry without watering. The best way to tell is by weight of the pot. If it feels real light it's time to water again. My plants seem to be loving this method. You cant overwater in one watering, it's about how frequently you water. I water this way and it takes about 4 days for mine to be ready, so I think yours will definitely be a little longer than that. Due to your pot and plant size. Weight is the best way to tell though in my opinion.

Hey bro, I've seen a few questions about ppm being high. I actually posted a question about it as well. Mine was 8000. My plants are looking good, but did a runoff test and I was so shocked to see 8000. I came to the conclusion it was my ppm tester as it wasnt a really expensive one. It works perfectly on the feed going in, but the runoff was sky high. I figured it didnt like the water going through the soil. You may have a similar problem. What I would do if I were you is do a mini flush sort of thing. Run double your normal watering through. Half plain water and half a nute mix. So say you normally water 1L and get 20% runoff water 1L plain ph'd water then a 1L nute mix. If I were you I'd probably feed at about 400-600ppm (.5 scale) as Roberts suggested. In my opinion I'd start a little lower and build up as you dont want to get nute burn. I'm no expert by any means but I've had this issue myself and I think your Ppm meter is being silly like mine was. But for the peace of mind before nutrients the plain water will help massively as you may have a buildup from your tap water. If you can get some purer water I think it will help the process massively as you have 100% control over what your plants are getting.

1800 is 3 times higher then ot should be in week 3.