In a broad sense, Thomas' advice is sound since he is advocating testing as you add acid. Its workable, but its not convenient or quick.
To clarify, the first time you adjust your water you need to go slow and careful. Once you've got your pH profile "dialed in" you don't need to be as careful. Mabrungard is correct that it's easier to just use a computer program to figure out mineral and acid additions - but only if you're certain that your water profile is actually correct. There can be huge seasonal variations in mineral counts for some water systems. Water reports only give average mineral counts and water analyses just give a "snapshot" of your water profile.
The reason I stress this is because where I live during the summer my city water system gets its water from a small lake (higher mineral counts) and in the winter it gets its water from Lake Ontario which is fairly soft. Also, what water you get depends on where you live in the county. Likewise, when I lived out West, seasonal snowmelt runoff could make the river water our system used during the spring softer than normal.
The statement that 1 to 3 tsp of lactic is suitable might be fine for Thomas' water, but cannot be applied to every brewer. There will be some bad pH consequences with that advice.
Absolutely. You need to know your own water.
In any case, the lack of perceived astringency is probably not a good indicator that pH was not a problem.
True. I was thinking more along the lines of chill haze/polyphenol extraction. I wasn't thinking of the effects of improper pH on enzyme activity. Improper pH could slow mash conversion, meaning possible starch haze.