If you are in soil, that's why you have never had to worry about ppm.
To find that sweet spot, soil growers can simply start off at a lower level of ferts, and ramp up until you see some scorch, and then let off a little. Record where that happened, and give just slightly less, flushing well every so often just to make sure that you don't get any salt buildup.
If you're still curious, there's lots of ppm talk in the hydro threads. Weedhound's grow log is very interesting.