The calculators cannot predict what will happen using the grains you are using because they only know about those grains in general terms. That is, a typical pilsner base malt has a titration curve that looks like the one on the 4th graph at http://www.wetnewf.org/pdfs/estimating-mash-ph.html. Your particular bag of malt will be somewhat different. So at best there will be variation (error) from the differences between the malt modeled and the malt being used. I say 'at best' because, AFAIK, none of these calculators use the complete titration curve in their models. It is very difficult and time consuming to get such data. If pressed I'll guess that the predictions are ±0.15 pH. Is that good enough? That's really up to you to decide. I wouldn't be comfortable with that but I think many would.
IMO, then, you really should obtain and learn how to use a pH meter. Others may disagree. I found that controlling pH, by use of a meter, made a pretty substantial improvement in my beers. For others this might not be the case.