Have you considered supporting OS V2.0? I read somewhere that is supports 5 expansion boards by default (48 Zones). Perhaps there is a way to ‘determine’ if the OS firmware is V1.8 or V2.0 and dynamically update the settings page.
If however it cannot be done then set the max expansions to 5 because when written back to OS 1.8 it will only set it internally to 3.
Ingo