Unfortunately, Kai's information is incorrect too. There is history behind that value when talking about chalk.
As you may have heard, it takes extraordinary measures to get chalk to dissolve in water. Unfortunately, that also applies to wort since wort doesn't have much of the 'strong' acid content needed to dissolve chalk. Brewers know that chalk doesn't provide the alkalinity that its chemistry says it adds and therefore the pH doesn't rise as expected.
With a -2 charge, you could expect carbonate to neutralize twice the acid content that bicarbonate can since it has a -1 charge (on an ion to ion basis). But carbonate does not dissociate completely in water or wort. To help account for that apparent deficiency, users assumed that the 158 ppm value should be applied instead of the 322 ppm value.
But in typical usage, chalk doesn't even provide the 158 ppm contribution to alkalinity. In fact, there is only a minor strong acid content in wort. Adding chalk to mash or wort typically only raises pH by about 0.1 unit...no matter how much chalk you add. While wort is clearly 'acidic' since its pH is below 7, the acids that drive the pH down are very 'weak'. They don't have the power to react with the carbonate in chalk.
The bottom line will always be: DON'T USE CHALK IN BREWING...IT DOESN'T WORK AT ALL!!!!!!