Probably doesn't matter much here, but 240.0.0.0/4 is hard coded to be unusable on Windows systems. It's in the Windows IP stack somewhere. Packets to/from that network will simply be dropped.
You can use absolutely any /8 that you want. I used 250.0.0.0/8 in my examples, but the FanNetworking wiki page uses 10.0.0.0/8. You can use any one you want. Have at it ;-)