When I first started hydro I fed half of what the chart for that product then used my meter to monitor the levels and if they dropped I would raise the EC next fill (every 7 days). I kept detailed records and when I started the next run I added the exact amounts for that EC. I did the same with the PH. As for rust spots, ya their a bitch and no it's not always caused by over feeding IMO. If you have unhealthy brown spotting then you should try to solve it as it does mean something is amiss, I was the same as I'm fuck it their ok lol, after doing some research with my particular setup and water (well) I was able to cure it and now plants are green till chop.