First, choose the style of beer you are brewing based on the color. This, when you add your water chemistry and grain bill, will help you nail the target pH of the mash. This is most critical part!
Then, you can also choose malty/balanced/bitter within that color range, and the spreadsheet will help you choose the flavor ions that will work best for that profile. Adding flavor ions usually also affects the pH, so whenever you add Gypsum or CaCl, double check the pH.
You can set up your water so that the mash water is different from the sparge water, so you can set the mash pH properly for mashing, and set the sparge pH to prevent tannin extraction during the sparge.
I really like the spreadsheet, but it will often feel like a juggling act when you are starting out. Eventually you will get a feel for it and it won't seem as "fiddly".
Personally, my water is pretty high in alkalinity, so I usually use RO water and add minerals for Pale beers, and dilute with RO AND add campden tablets (stupid chloramines!) for very dark beers. I usually have to add some amount of acid (I use Phosphoric) depending on the beer color and my water dilution.