The spreadheet is (mis)used by many based on the shaky premises that beers "require" a certain RA, that the required RA is dependent on the color of the beer and that adding chalk to brewing water at levels that will not dissolve or to the mash can be added into the RA equation. The actual calculations are based on the premise that it takes 3.5 mEq of calcium (or 7 mEq magnesium) ions to "neutralize" 1 mEq of alkalinity, that the malt is exclusively base malt and that this base malt has essentially the characteristics of the lager malts studies by Paul Kolbach prior to, during and just after WWII. You could certainly add to this list the assumption that mash thickness would have to be nominal for the numbers calculated by the spreadsheet to be meaningful. But as there are so many other assumptions underlying the spreadsheet I think you can safely omit trying to model thickness as the "error" incurred by not considering it would probably be inconsequential in comparison to some of the other things that the model ignores.
If you add acid or acid malts or calcium/magnesium salts to a mash obviously the more water you add the more you dilute these and the less their effects at lowering mash pH will be. But remember that the mash is a phosphate (at least - lots of organic ones too) buffering system which will tend to resists mash pH change to some extent.
Where the spreadsheet is really useful is in determining the RA of the water by itself both before and after you doctor is up with minerals. If water sample 1 has higher RA than water sample 2 you will need to add more acid (in the form of mineral, inorganic, dark malt or acidulated malt) or try to get more acid by addition of calcium and magnesium than you would if you brew the same beer with water 1. If you do not, then the pH of the mash with water 2 will be higher.
I advise using the spreadsheet to calculate the RA from the specs furnished to you by your supplier and then decide which styles of beer might be suited to water of this RA. You can use the chart on my website (http://www.wetnewf.org
), which plots RA vs. effective hardness, to help you do this. Unless your water supply is strange in some way (e.g. very alkaline, very hard or very briny) it is probable that you can make most beers with it without tweaking it. If it is strange it's probable that it can be fixed by decarbonation or dilution with RO or other low ion content water. Salt adjustments, other than the most simple ones, are best left to experienced brewers. If your water is extremely soft then you should add some calcium chloride. If you want assertive hops, then add some calcium sulfate as well. But don't let the spreadheet, or anyone else, tell you to add tablespoons of calcium carbonate to your water because you want to brew a stout of 40 SRM (80 EBC).
As for mash pH, the best way to control it is to check it as you brew and adjust, with acid, calcium salts and/or chalk, in the mash tun, if it is out of range. It is unlikely that your pH will be too low.