I need help understanding ppm/tds/ec

I need help understanding the ppm readings. I have a hanna ph/ppm meter and when I dip it in my tap warped it reads 300. If that's a cumulative number I don't understand how that helps if you are using more than one mineral. For instance, if the recommended flowering ppms were ideal at 1300, couldn't you just add Fallacy+ until it reads 1300? And if so, how is that helpful? How do you know which nutrients need adjusting? I pretty much have the expert AN bundle and when flowering begins, I will have at least 3-4 different nutes going in. How do I know if I have too much of too little of each?

Haha my phone autocorrected CalMag+ to Fallacy+.

Like you said adding 1300 ppm of cal mag would not help. Keeping your nutes in the ratio they are supposed to be used together and keeping that under or at your desired ppm is what the measurement is good for.

the measurement does not tell you which nutes to use, ony how much your plant can handle. It is up to you which nutes to add and how much. I usually start out at 1/4 - 1/2 of what the bottle recommends, and work my way up to around 100%.

Awesome, very much appreciated.

typically, the way i do it is, fill my tub with water and let it sit over night.. then check the ppm. normally around 160 for me being in the city. then i get my gh nutes which have recommended feeding schedule /desired ppm. for ex i am in earlier veg state of a few weeks so i have bumped my nute ppm to 400(n-p-k). once i add the nutes, i again check the ppm (normally about 550ish because i am only focused on the nute ppm of around 400 for my n-p-k). once that is all taken care of i put in 1 tsp/gallon cal mag (maybe even slightly less), then get my ph stable 5.8-6 and am set to go. i don't check the ppm once i have mixed in the nutes and make sure i have the desired nute range. i do check the ph every 12 hours to keep it as close to 5.8-6.0 as i can w/o messing w/ it too much. maybe i am doing it all wrong, but it is working well so far.
i guess in flowering when the plants start to drink lots of water, it is important to check the ppm due to loss of water. you could be 1000 ppm when you mix nutes and 2 days later once the plants drink some, the ppm could jump to 2000 because plants are drinking more water, less nutes etc. so you need to dump in proper ph water maybe w/o nutes to get the ppm closer to 1000 again.
hope this makes a bit of sense.. checking the ph/ppm can seem intimidating but gets easier as time goes on and you start to get a feel for things

one other note. i normally don't worry about the ppm calmag adds because i like to sit on the lower side of the nute schedule rather than the high/hot side.. to me less has been more so far. it is easier to add a bit of nutes to a slightly deficient plant than burn the crap out of your plants and try to correct that.