All the reading, books, videos, classes, and information you can absorb will not replace years of experience. That being said, you will learn over time exactly what to feed your plants, and when.
Keep up the hard work, don't be afraid to make mistakes, and try to learn from the ones you do make.
The way I see it, if your plant is starting to show "signs of malnourishment" or deficiencies, it's already too late. Leaves that show deficiencies will not magically turn back to normal- the damage is already done. The key is to prevent it from getting to that point in the first place. Only hands-on experience will teach you what you need to know to succeed, and every strain is different, so stick with what you know and don't be afraid to learn new things when dealing with new strains.
For example, my Forbidden Fruit strain is one of the hardest strains to grow. It's a medium yielding plant and is extremely picky about what you feed it. I believe for this reason alone, is why many growers have stopped growing this wonderful strain, because they didn't like it's yield output, or they couldn't figure out how to feed it properly. It took me a couple of seasons to fully master Forbidden Fruit and I've been growing weed for well over a decade.
Hopefully that advice helps keep ya motivated - don't give up! Experience + Patience will always = Success, if you want it bad enough!