Disk arrays come in multiple possible configurations. You would need to determine how many disks you will be using in the array and the configuration you are using.
Some configurations have zero tolerance for any failures and hence are actually less (potentially significantly so) reliable than a single disk, others can handle a single disk failure, others two disk failures, others ... well let's just say it depends where in the array the second (or third, or fourth, etc.) disk fails.
You would need to look up the formulas for each array configuration you are looking at.
I did break this all down (or at least find the links) about five years ago, but unfortunately the sources I use escape me at the moment. The calculator linked in the link you posted above is useful enough to have some utility if you want a purely mathematical calculation for the two specific array configurations it covers.
It's actually explained in the link you have, but as you can see, disk array availability is rather complicated. Calculating annualized failure rate (for a single disk) or mean time to failure is more simple and is sufficient for most. If you are still intent on calculating array availability then you may want to ask why you are doing so, and understanding the mechanisms that affect it.
aka find some links that explain things with better examples than the glossary in the page you linked?