don't forget it's logarithmic so trying to treat it like it's linear won't work so well.
my mash pH was a little low (4.9-5.0ish) so I used bru'n water to guess how much baking soda to add. I added too much b/c I forgot that I was operating on a log scale. I was probably up close to 6.0 for a few minutes until I scrambled and added some lactic back in to knock things down to probably 5.7 or so. then I added some more gypsum and calcium chloride to knock things back down to 5.3 (phew!).
by this time I had lost 4-5 degrees on my mash temp from all of the opening of the kettle and mixing. so I pulled a little over a gallon of wort/grain and brought it just to a boil on the stove and added it back in to recover my target mash temp of 150F.
hopefully that brief time spent at a high pH didn't cause any problems