The nutrients you are using will come with recommended dosages and the company that makes them/sells them will offer a feed chart which should state the EC/PPM requirements at all life stages. Just google "advanced nutrients feed chart coco".
I use EC rather than PPM but my understanding is that 1 EC = 700 PPM... autos need less than photos, but my photos are currently taking about 1500 PPM in hydro and they are roughly at the same stage of (a longer) flowering period. So 900 PPM is probably about right (but I do not use AN myself, so it's best to check their recommendations).