nvSRAM- Storage capacitor selection

**broken link removed**

I've a question on selection of VCAP for nvSRAM. On What basis is a capacitor value decided for a particular nvSRAM density?
I was going through this document from cypress (see link above) and in page 2, under 'key characteristics' I came across values say, 68uF for a 4 MBit parallel nvSRAM? HOw is this 68 uF arrived, is there some formula?
I'd be happy if anyone could help me through this.

It's likely determined empirically when they initially test the device. They look at the voltage drop on the capacitor during the shutdown AutoStore cycle and select a capacitor value that maintains sufficient voltage during this operation with adequate margin. The value given is the minimum recommended capacitor size. It can be larger if you like.

Thanks! That was concise..