ipnat configuration

[Travis Garrison]

I am working on an ipnat configuration to replace our aging Cisco FWSM. We have several private IP subnets that we would like to map to several public ranges. There are more private ranges so we would actually be doing a pat. The question is on how to configure the public ip ranges so that we can use all of them in a big pool. Currently our Cisco does a 1 to 1 NAT until it runs out of addresses and then filles the rest through a single PAT ip address. We would like to do a round robin PAT and only PAT a handful addresses per public ip address. Do we need to use ippools for our setup? 

I am thinking of something like this 

ippool - public side 
64.x.x.0/23 
69.x.x.0/24 

ipnat 
map em0 172.20.30.0/24 -> ippoolpublic 
map em0 172.20.31.0/24 -> ippoolpublic 
map em0 172.20.32.0/24 -> ippoolpublic 
map em0 172.20.33.0/24 -> ippoolpublic 
map em0 172.20.34.0/24 -> ippoolpublic 
map em0 172.20.35.0/24 -> ippoolpublic 
map em0 172.20.36.0/24 -> ippoolpublic 
map em0 172.20.37.0/24 -> ippoolpublic 
map em0 172.20.38.0/24 -> ippoolpublic

Thank you
Travis Garrison