If you brew with DI water
If you use Maris Otter as base malt
If you use 10% roast barley as the sole dark malt
Then you will get a mash pH of around 5.5
If you mash with water of modest alkalinity then your pH will be slightly higher.
If you follow Ashton Lewis's Irish Stout recipe (pale ale, 10% roast barley, flaked barley) as given in Michael Lewis's Stout monograph with modestly alkaline water you will get about the same pH.
If you use DI water with 20% roast barley you will get a pH close to 5.35
If you use DI water with 30% roast barley you will get a pH of close to 5.2
If you do anything else then I don't know exactly what will happen but common sense and experience (mine and others) dictates that if you keep the roast material at something close to the levels mentioned here you will probably get similar pH's.
If I put my (Ashton Lewis's really) recipe into Brun Water it predicts mash pH 5.1. I measure 5.55 every time I do this beer (± a couple of hundredths).
Martin puts a Brown into the same spreadsheet and calculates and then measures 5.0.
Bit of a discrepancy here and unfortunately it is up to you, the brewer, to decide what to do. There is really only one answer and that is not to rely on Primers or spreadsheets for more than general guidance. Buy a pH meter and use it in every brew. You will not be sorry you did.
It seems more likely from feedback here from those who have done this that you will have pH that is too high, as most beers require some acid, than low. The exception, of course, could be in dark beers, like stouts, where the dark malts can contribute quite a bit of acid. This is why the Primer does not advocate the addition of acid to stouts (or other dark beers) but does for other styles.
The Primer does not pretend to be all things to all men. It it intended to get you started and will, in most cases, get you a good beer. It really wants you to get started measuring mash pH (and the pH for other parts of the process too). I'd be willing to bet that you'd be just fine with only a CaCl2 addition but of you are using over 30% roast barley in your stout (and people do things like that) your pH could go below 5.3 and some alkali would be warranted. Even at 10% you could have low pH. I've seen Maris Otter turn in a DI mash pH of 5.6 and I've seen it turn in 5.8 (different maltsters).
If you decide to add alkali you won't ruin your beer. It just won't be as tasty as it could potentially be. And of course the same applies at the low end.
But if you really care, why take chances? Decent pH meters are available from most LHBSs at less than $100. I always point out that a pint at a good brew pub costs about $7 around here and recommend that you just brew 100/7 more pints to cover the cost of the meter.
I'll finish with A.J.'s first cannon of brewing: Never add alkali to brewing water nor to mash unless a measurement with a properly calibrated pH meter says you need to.
It's the first cannon because statistically (no formal data here of course) it seems excessively high pH is more likely to rear it's ugly head than low. If you follow the first cannon then, of course, you will have the meter to hand and can (and should) check whenever acid addition is contemplated.
Note: I do not (AFAIK) own stock in any company that manufactures or sells pH meters.