I try to keep it as simple as possible by following the steps below:
-I send my water out to Ward labs every 6 months (so far the results have almost been identical)
-I have a set of water profiles for each style of beer
-I plug my source water and my target water profile into EZ Water Calculator. http://www.ezwatercalculator.com/
-Play with distilling the water down and modifying salt additions until I get close
Thursday I'm brewing a Cream Ale which is going to take 50% distilled water but I got close to my target profile. My source water sits in the middle of light and dark styles. The Cream Ale is at the extreme light side which requires that amount of distilled but I usually find I only need 10% or none at all for most of my beers.
The biggest difference I've noticed is how my hops shine in my hoppy beers. I attribute this to the chloride/sulfate ratio in my profiles which will determine your malt/hop balance.