I use "Bru'n Water" but also "Mash-made-easy", a freebie by this character:
@Argentum . They are very different, but "Mash-made-easy" is more up-to-date and supported very enthusiastically by the (American) author. Both have their oddities that you need to get used to.
All calculators should come up with the same results for adding non-alkaline salts (e.g.
not sodium bicarbonate) but can differ with pH predictions. At the moment, for me, "Bru'n Water" gets closer, but that should change with my wappy water supply (presently it is at least being fairly stable). I have Graham Wheeler's profiles keyed into Bru'n Water. It is worth remembering
your water supply might work better with a different calculator than the one you are using.
The good thing about calculators is you don't need detailed knowledge of "alkalinity". "Alkalinity" is an important concept to understand if you are building your own calculator or calculating additions using just a standard calculator (or slide-rule, pen & paper, etc.). If "Alkalinity" is over your head (hardly an uncommon thing) just use a water calculator and have it figure out additions to get the right mash pH. Or wing it, or do nothing, most* water will settle to a correct mash pH though you may be limited to best results for just a type of beer.
* Most? At the moment mine settles out at <pH 5.0 . It doesn't seem to fall into the "most" category.