[OpenIndiana-discuss] Formula raidz storage space caclulation

[Randy S]

The information that I have about the formula to use is (n-1) x storage space per disk
n=number of disks 
1=is de disk used for parity in a raidz1 configuration.
In the case of Raidz2 , this number would be 2 (n-2) x space.

My question is: is this formula correct?
Reason, my outcome always differs with from the outcome on the following website: http://www.servethehome.com/raid-calculator/
What would be the correct formula to use?