No, it's not stupid. Better to ask than waste an entire grow.
When a nute company's feeding schedule calls for, say, 500 PPM, they mean 500 PPM of their product. If you start with tap water of 350, then you'd need to add that to the total - so if they say use 500 PPM of their product, and you're starting with tap of 350, you're shooting for 850 as the final number.
Where you want to start being careful is if you're starting with tap water that already has a lot of dissolved solids, like 350 or 400 - especially if you're adding supplements like calmag, because you don't know what those 350 parts per million are. A lot of people seem to assume that it's a roughly equal mix of calcium and magnesium, and there've been a lot of posts here that seem to be based on that false assumption - no offense, because they mean well, but there's a lot of misinformation in some of those posts. It completely depends on the geology of the region in which you live.
For example, if you live in an area where the geology is primarily limestone, most of the dissolved solids are probably going to be calcium carbonate - with no magnesium at all, or possibly just trace amounts. It's quite possible for someone in a limestone region to see magnesium deficiency in their leaves, add a full dose of calmag, and correct the magnesium problem - but boost the calcium content way beyond what it ought to be, and in some cases cause lockout or even worse. I learned that once the hard way when all the phosphoric acid in my PH down, plus the phosphates in my PK boost nutes, combined with the calcium carbonate in my water and the calcium in my calmag and formed calcium phosphate, which precipitated out as a non-soluble white substance that turned my res cloudy and robbed my plants of both calcium and phosphates.
For a new hydro grower, it can be very helpful to think about nutes and dissolved solids from an entirely different frame of reference. Don't think of them as nutrients - just think of them as chemicals. Every time you add one to your water, you are creating an entirely new chemical compound - and every time you add another nute or supplement to that compound, you are creating still another compound, which is going to react in a very specific way to whatever you add to it next. Temperature, aeration, PH Up or PH Down, even the sequence in which you add each ingredient can all have an effect on the final product in your reservoir.
That's one reason I like to keep things as simple as possible - I try to add only what it needs, when it needs it, and only as much as it needs. No more. One of my mottos is, "When in doubt - don't." You can always add some more later, but it's pretty difficult to take it out again without dumping the res. I try to look at PPMs as a bank account in reverse - I try never to waste any extra PPMs, because if you dump too much shit into the tub, it's surprising how fast you can hit 1500 PPM without even knowing you were coming anywhere near it.
