Whoops! It's been a long day ....
Yes, of course, you're right, so my explanation wasn't as sensible as I'd have liked ...
But I still think my answer is right:
10mm centre-to-centre spacing would leave just 1mm from the centre of the B string to the edge and 1mm from the C to the edge, which is clearly not feasible (especially since the B string will be about 3mm wide). With 9mm spacing, that leaves just 3.5mm on each side, which sounds about right, or maybe a bit tight, which was why I reckoned it would be a shade under 9mm.