Richy,
It wasn't the time part I was uncertain about, but the relevance of the different 'type' of night w.r.t. night flying.
I agree with your calculation, but you have taken the earth's inclination and latitude into account - the further north you go in summer the less night you get (ie sun below horizon for less time), until you get north of the artic circle, when at midsummer you get no night at all.