I took the liberty to fill in the numbers for your water, set the units to lb and gal and made an assumption about the amount of water and grain you’ll be using.http://braukaiser.com/documents/misc_forum/Kaiser_water_calculator_for_dean.xls
When you enter lactic acid (88% or acid malt) as % of water the water treatment becomes independent from the actual water volume which might be helpful.
I assumed 10lb of grain and a strike water volume of 4 gal. This gives a mash thickness of 1.6 qt/lb. To estimate the mash pH the spread sheet needs to know the mash thickness in addition to the beer color and residual alkalinity.
Once you enter the beer color you get a mash pH estimate. That estimate should be between 5.3 and 5.6. To make a 5 SRM beer with your water I can add 1% of the water’s weight as acid malt. This drops the residual alkalinity to -190 ppm CaCO3 and gives a mash pH estimate of 5.3. I also added 100 ppm gypsum to boost your calcium and sulfate levels.
If you want to dilute the water with distilled or RO water, enter the percentage of your tap water into E18. If you choose 50/50 enter 50%. Now cut back on the acid malt and lower it to 0.8% of the water weight. That gives you about the same mash pH.