Basic dummy iptables SNAT multiple IPs algorithm question

Hi guys,

I've been using iptables and some of the modules it comes with for a while now but only recently as I stumbled into a big NAT444 setup, did I reach the point at which I'd like to find an answer to a theoretical question.

I wanted to ask:
If I have a rule that says:
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.0.0/16 -j SNAT --to 20.20.20.1-20.20.20.22.254

Then how are the IPs from the private pool distributed to the ones from the public pool in the two following cases:
- with persistent
- with random

Is it a simple mathematical thing like:
if private pool has X IPs ( 65536 in this case) and public Pool (Y) has let's say 766 IPs in theory => X / Y = aprox 85 private IPs per public IP and then:
- 192.168.0.0-84 => 20.20.20.1
- 192.168.0.85-169 => 20.20.20.2
and so on ?
or does this depend on the number of IPs which are online and the algorithm is a bit more complex ?

Also, what happens in case random is being used to the same algorithm ?

Hope I'm not troubling you too much with theory related questions.

Thanks,
Mihai