Is there a PPM I should be shooting for? Is there a number I should work towards so by flowering the plants are taking in the maximum? 1,500 by flowering? Higher?
A bit depends on the nutrient line you are using but in general 900 ppm in veg and 1,100 to 1,200 ppm (NACL scale) is close to ideal. If running nutes in media like coco it pays to monitor the runoff TDS and adjust as necessary based on runoff numbers.
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Really depends on alot of factors.
In soil useing general hydroponics trio.
I would hit somewhere around 1000ppm (useing the 500 scale) at the highest.
Now that im in autopots useing coco-coir and perlite, my ppms are hardly ever over 700ish 
Things like nutrient lines, substrate, watering habits play a roll.
It best to follow manufacturers suggestions at first or slightly less even. And once you gain a feel for it, increase untill plants respond, if response is negative, reduce feed, if response in positive then carry-on 
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I start with 1/2 recommended dose at first and mid flower I give em a bit more than recommended. Usually bout 1000ppm.
Keep in mind that if your plain water tests high you’ll have to account for that and dial back the nutes
Example tap water @350ppm needs a mere 650ppm added to reach the 1000ppm mark.
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My tap water is over 500 ppm and 7.7 PH. That’s after a water softener. I just got my RO system so that’ll be nice when I get it installed. There’s 7 different filtration devices plus UV light on this. My water will be as pure as water gets soon. 100 GPD is plenty for me.
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Something folks have to deal with is base water TDS has to be factored in to your nutrient load. This is why so many growers use R/O as they can hit the plant with known salts in known quantities. My muni water in California was over 550 ppm and was basically impossible to use for hydro. Here in Colorado it’s 120 ppm so I don’t worry about it.
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My tap is close to 400. I shoot 11-1200ppm. When using RO, I aim for 950-1050.
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Damn, my tap water was 83 ppm once. It’s usually closer to 70. A lot of restrictions around our water source. No boats with combustion engines, no livestock within several miles, etc. Plus it’s basically in the middle of nowhere. I’d almost drink straight from the lake. Almost.
I do the opposite of what someone said earlier. I deduct the EC of my starting water from the EC of the nutrient solution. I do the same for pH up or down. I actually check EC before adding it. If it’s not nutes it doesn’t matter in EC. I guess that’s why the same source that said do it that way said don’t use tap water over a certain EC.
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So the tds thats giving the EC reading is minerals of some sort.
And nutrients are minerals so the EC in tap is nutes, the resaon it gets tricky at higher ppm is those minerals/ nutrients are unknown. Typically people do not know exactly what minerals are creating the ppm in their tap.
Typically things like calcium can be high in tap or iron, or anything really…
But those ppm are the same minerals that we call nutrients 
True. Dr. Coco says don’t use tap (actually any type of) water over 300 EC. I guess under that EC there aren’t enough minerals to throw off the nutrient element ratio (NER) of your nutrient solution. I’ve heard you can get a report from your local water authority that tells exactly what’s in your tap water. At 70ppm I haven’t bothered requesting one.
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.6 EC or 300 ppm NACL scale…
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I was thinking 300 ppm as well. This is an excerpt from cocoforcannabis, “You may or may not be able to use untreated tap or well water to mix your nutrients. The key consideration is the EC. Starting water must be lower than EC 300. The lower the initial EC the better. Using distilled water or water filtered through reverse osmosis is best. However, tap and well water may be used if they start less than EC 300/0.3.”
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Was reading an article on RQS and they said 300 ppm was the cutoff for tap water. It even said you can go up to 500 ppm if there’s old piping in your house. I don’t think I’d risk it over 300ppm. I guess it just depends on who you ask.