I note that OpenStreetMaps [OSM] i.e. the tile server linked in by the XML file shipped with the NaviComputer application is now trapping the use of zoom levels 17 and 18. I think there was an issue before but OSM relented or Markus got something changed. OpenStreetMap tile server is also rejecting zoom levels 17 / 18 on Mobile Atlas Creator. If you read OSM terms it does say that it is prohibited, so somewhere along the development line they have closed the back door on this if it was left open. The server indexed within the XML file is only allowed for levels up 16 else users down loading tiles hog the server for editing.
The link below provides alternative tile services which use the tiles as published on the OSM wiki:
http://wiki.openstreetmap.org/wiki/List_of_OSM_based_Services
So use of HTTPFox on some of these allows you to find alternative tile servers which give you zoom levels down to 18. The syntax is easy enough to edit into the XML file. Some of the servers are a bit slow whilst some are just as snappy as the OSM edit one. Some change the rendering from the OSM giving a choice [CloudMade] whilst others show the default [Geofabrik]. There are other interesting links on there, as well to alternative maps which may be of use.
The developer doesn't seem to be around recently so with spring in the air and the great outdoors beckoning for rambling one needs the 17/18 zoom levels on your device. Also I'm not convinced that the Navicomputer Blog is being fully serviced e.g. post are getting put up. Thus discussion may be a better place here and I suspect users may be moving towards self help.