I've just said this to you in an email Ian, so forgive me for saying it here too, but everyone can read it then, and I've seen your drawings, which they haven't.So Ive done a sketch based on a #6 Bock Nib and Housing and my thought is that its going to be a fairly chunky pen with cap diameter of approx 15mm. This is based on a 12mm tap for the cap / body.
Im thinking the #5 may be a better option to give a bit more flexibility in shaping the section and allowing a bit more wall thickness when drilling / threading. Would be grateful to hear the thoughts of others in this regard.
A 15mm cap is not big by any means. You are possibly just used to seeing size 5 pens, which makes you think that 15mm is too big. Your drawing shows a 1mm overlap all round, which is just fine in my opinion. One of the most popular commercially available fountain pens on the planet is a Jinhao X750, which in itself is more or less a direct copy of a Pelikan, which conforms more or less exactly to your outside diameters, so I really wouldn't worry about it. You are on the right track.