Garden room - framing question. 400mm on centre.

[gsf600y]

I've got a question about framing the walls of a garden room.

I'm just sussing out how I'd go about doing the framing.
Here's my basic design I'm playing around with, which is an odd shape due to garden (no idea how I'll do the roof yet):

From what I understand, the 400mm vs 16" question just seems to come down to either making the interior plasterboard easier (400mm), or the exterior OSB (16").

Personally, I prefer to work in metric.
What I'm confused about is if I've chosen 400mm, where does the 400mm begin and end? If I'm hoping to get full sheets to fit, would it not make more sense to have the 400mm starting on the inside, as per the RED measurement?
Or do you measure from the outside still, as per the blue?

Any tips appreciated.
Thanks

 

[big-all]

do vertical spacings at 400mm centres and horizontal at 406 or visa versa and sheet material will both work in there own orientation
you need to work out iff say six or eight foot high or 1800 0r 2400 high is what you want
in general metric height is more economic with imperial length as metric height is smaller by a few inches