Micrograms are 1/1,000,000 of a gram and frankly if you can measure w/in a few millionths of a gram with your magic tsp more power to you. Are we not trying to do something extra special when we fix our water? Go to the 'next level' in brewing? Why bother if your not going to weigh the salts out... Check out one of these scales for under $25 that is accurate to the .01 and don't kid yourself about tsp accuracy. If you want consistency you have to weigh stuff out, you're better than a tsp go the extra mile
As for what the spreadsheet is for, it is to help you figure out how much salt you should add. It doesn't take the place of testing your water before or after the addition of salts. If you add the correct amount you should be at the value the spreadsheet predicts. Depending upon how serious you want to get you can purchase some relatively inexpensive tests for at least the crucial part of the water. First get your water AG tested at Wards. Then use these crucial test kits to make sure you water is still where Wards said it was BEFORE you add salts, then add the salts and then make sure you hit your mark. I totally understand that people don't have labs full labs but you can get chloride, sulfate, total alk, total hardness, magnesium, and calcium test kits and easily calculate your RA and your Cl:SO4 ratio. (I'm crazy and I do this) Again, only if you are want to go for extreme consistency in your product. It would matter in some beer styles more than others also, bitters as an example. If I'm going for a 1:10 Cl:SO4 ratio I want to know I've hit it or gotten really close.