I’d recommend using the EC reading instead of PPM, because there are at least two different scales for conversion of EC to PPM
Because EC is the actual measurement that PPM is being calculated from.
1.0 EC can be 500 PPM or 700 PPM.. depending on the scale used by that company or grower. However - most do use the 500 scale.
As far as a rate of feed.. I have run autos, but don’t anymore because they are finicky as hell at times (among other reasons). BUT I would recommend around 1.0 (500ppm on the 500 scale) as a good medium and up to 1.4-1.6 (700-800ppm on 500 scale )in flower if it appears it can take it. I’ve fed 2.0 in hydro with autos in flower and that was a bit aggressive and gave some toxicities.