Ok, so I tried to create a Macro and Button, and I was getting close, but I've had one too many homebrews, and I don't really care anymore. Solver really is very easy to use. I'm attaching the spreadsheet sans examples, so feel free to use this. Again, this spreadsheet is extremely versatile: you can input any initial water conditions and input any target water conditions. Through Solver you can add any constraints such as integer units (my default), maximum amounts to add (10g my default) any target ion concentration, target alkalinity, and/or target hardness. Again, these extra constraints are totally for your own use. I've set it up so you don't have to fiddle with Solver at all.

In this new spreadsheet all you have to do is input values into the Bold Yellow boxes (your water supply and the target water). Assuming you have Solver installed and Macros enable, click on Data, then Solver. In the pop up box click Solve, let it think for a few seconds, then click ok. You now have the optimized results! The light blue cells indicate the cells used by Solver, but you shouldn't need to worry about these as the solver macro is pre-configured.

If you're having problems using Solver or would like to learn more about it, try this link: http://office.microsoft.com/en-us/excel/HA102190021033.aspx