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. I hope you have four hours to spare and then another few to digest.
Here's a slightly easier way. Go to the spreadsheet linked in my sig, enter zeros in all the water stats because that's pretty much what distilled water is. Set your mash and sparge water values to your recipe.
Select various amounts of chalk, gypsum, epsom salt, etc (skip the acids) until you're close to say the "london" profile, except I'd lower the sodium. Here's one way of going, note these additions are PER gallon of water used in the mash. Stir them into the mash. Of course I'm assuming 100% distilled water.
Adjustments (grams) Mash
RA (mash only): 85 (12 to 17 SRM)
Cl to SO4 (total water): 0.62 (Bitter)