Which stage in the mash is pH most important (15/30/45/60 min?)

The reason I am asking is that I am wanting to hold off on adding some chocolate and crystal 60 until I mash out. I'm doing a 2.25 gallon BIAB Nut Brown with 100% distilled water. The recipe is a scaled Lil Sparky Nut Brown:

3.3 lbs Marris Otter
0.4 lbs Oats
0.2 lbs Victory
0.125 lbs Chocolate
0.4 C60
Fuggles and EKG

I would mash the MO, Oats, and Victory. If I do that, and add the following salts, then I would get a mash pH of almost 5.6.

1.0 gram Gypsum
1.0 gram CaCl2

I would then need to add 1.25 mL of Lactic Acid to get it down to about 5.4.

However I have read where mash pH trends down as time goes on. Also, by adding the c60 and Chocolate later, that would drive the pH down even further. I'm worried that by the time I get to the end of the mash, my pH will be too low going into the boil. Which btw, how does kettle pH come into play here? Does pH keep going lower during the boil?

In the end, at what time point should that pH be at 5.4?

Any recommendations on additions?

 

Slightly Off Topic here, but how is the tap water in Birmingham? I live in South AL (Daphne) an our water is very soft. Just curious as to why you're using distilled water.

 

My water comes from a different source than the rest of bHam. Were on the west side, or Mcalla area. Here is the water report. High Alkalinity is the reason I went to RO/Distilled. That, and just the ease of building your own water from scratch and taking the guess work out of it.

pH 8.5

Total Dissolved Solids (TDS) Est, ppm 266
Electrical Conductivity, mmho/cm 0.44
Cations / Anions, me/L 5.1 / 5.6

ppm
Sodium, Na 38
Potassium, K 3
Calcium, Ca 61
Magnesium, Mg 4
Total Hardness, CaCO3 169
Nitrate, NO3-N < 0.1 (SAFE)
Sulfate, SO4-S 9
Chloride, Cl 18
Carbonate, CO3 16
Bicarbonate, HCO3 244
Total Alkalinity, CaCO3 226
Total Phosphorus, P 0.16
Total Iron, Fe < 0.01