Question about configuring an lxd fan network


I have read documentation on how to use LXD with MAAS,

What I am wondering is if there is any way to properly configure the lxd overlay fan network, e.g. described here
How would defining a subnet for such a network look like, as we know the gateway part would be different depending on the machine being the host for a vm using this network, also the CIDR would differ,as the second and third octet would be defined by underlay parameter, so I guess the GW should be empty? What should be the correct CIDR?

@billwear maybe I am missing some part of documentation regarding such a case.

Thank you in advance