This depends on how the Outdoor AP is setup.
Is it setup to extend the existing network, or its setup as a router itself, and is creating a secondary network outside?
If its just extending, it can be anything you want really in the 10.0.1.x range. Just make it 10.0.1.2. The only real requirement is that they need to be on the same subnet unless your outdoor access point is acting as a router itself creating another network. If that is the case, the IP address facing the AirPort Extreme should be one that is in the same subnet as the Airport Extreme. And you'll need to make sure the address you assign to it, is reserved in the Airport Extreme, so i does not try to assign it to any other device.
The Outdoor AP can then issue any IP addresses it wants to the devices connected to it, and the IP it has that is facing the connected devices can be anything within the subnet it is handing out.