are you asking in terms of aesthetics or strength?
I've always (going back 15 years of A-headsets) kept the stem-clamp covering not less than 3/4 of the head tube. that's by no means scientific, but I've never had a failure (steel forks always, on mountain-bikes and very rough terrain).
Once you've measured fork axle to stem, as a baseline, I think it's great to play about with options. It's also one of the great things about the bikes we have (ie steel), that they're not shrink-wrapped and you can play around with different configurations more easily.
(btw, I had two tall spacers on my CT which were too tall for my new (oversized to fit the new bars) 'flipped' stem. So if you want a large (Thorn) spacer I have a 50mm gathering dust).
cheers,
Doug