Honestly, I don't put much stock in height measurements simply because I've never found an action that is too low at the nut to be comfortable. Therefore, my approach is to get the slots in the nut as low as possible without buzzing, then bring the saddle down until the uke is comfortable to play up the neck and appropriate to how you play (typically a little higher if you do a lot of picking, a little lower if you're strictly a strummer). If I can't get perfect intonation at the first couple of frets without buzzing, then it's time to level the frets.
The business card under the string at the first fret thing is a pretty meaningless measurement in my mind - it's not proof that the string is low enough for good intonation at the first fret nor is it evidence that the action is as low at the nut as it can be. The latter is important because typically, even after good intonation is achieved, on most ukes you can go a little lower yet without buzzing and that makes the uke easier to play (especially for things like the Bb chord, etc).
John