What Ph should I water my Autos at? Considering this substrate and the previous problems I had with it ?

Hey, So that bag of medium would be classified as soilless, it is a mixture of coir and peat with perlite. With that in mind a PH your water to a range of 5.5-6.2, and you will be good for liquid nutrients. A ph of 6.4-6.8 is need for dry amendments like Gaia Green. Some of the overfeeding you may have experienced in previous grows could have been from the concentrations being high and the slow release already in the medium. You should be measuring your liquid feeding solutions in EC or PPM, and also measuring the run off if using liquid nutrients. This way you will know how strong it is. If to many salts build up the in medium it locks out nutrients and causes issues with the plants. Most bottle labels tell you how much to feed for a bigger plant and are feeding heavier so you use more, you can be feeding up to 1/2 to 1/3 or even 1/4 the suggested amount and be ok. Autoflowers and Photoperiods have different feeding guides as well. In general Autos like a bit less feed as they are sensitive. As for the feed in that medium. it is slow release and will only come out in warmer weather and when watering. I would try and get a medium that does not have that so you can control the feeding with your nutrients and avoid any issues. Good Luck and Happy Growing!