I use Palmer's spreadsheet which is available at the bottom of this page.
If you decide to go with brewing salts, which you probably should if your water is almost the same as distilled, you will be adding you first addition of salts directly to the mash and then the second addition not to the sparge water but to the boil itself.
I would also recommend listening to the water series from the brewing network. Lots of good advice there if you have time to listen. Here's
the link to that.