Personally, I think that having site-specific namespaces should be at 100-199. This allows them to show up earlier on "special:allpages" or any other places that lists namespaces by number. Then place all of the extensions.
Or maybe this ...
0-99 - MediaWiki 100-999 - Site stuff 1000+ extensions.
Extensions can reserve a group of 10 (5 namespaces). For most extensions, that should be fine. It is only big ones that have a whole group of extensions that it might make a difference and they need more.