Palle-
It sounds like you have thought out the rudder area and design issues quite completely.
I don't think the rudder area is extremely critical, so if you use an 'average' value you should be OK.
Anyway, you can always build rudder ver 2.0,3.0 etc... ;-)

Some other things to keep in mind are the rocker of the hull and the position of the rudder.
On a surfski, the standard rudder is usually not at the extreme stern, so has less 'leverage' than a stern-hung rudder, so the area may need to be greater. (??) On the other hand, a stern-hung rudder may spend more time out of the water..
How easy is the boat to turn without the rudder?
The Panthera (which shares some characteristics with the Njord) is quite maneuverable without the rudder, and the rudder is used to make it 'track' better, IMO.

Have you completed your Nomad? The Najad looks similar, but its plumb stern would make attaching a rudder with a bolt into the end plug easier, so perhaps changing the stern shape would be something to consider, if you can. The alternative -a metal bracket- isn't as attractive, IMO.

If you can get a good look at the SmartTrack rudder (rudder post and tube), you can get some ideas for your DIY rudder.