There are some impressive, outstanding Spitfires showing up here!
Rob: your Mk IXc is fantastic; those colours really go together well, and the way you've accomplished the underside with the panel lines and weathering makes it really interesting to look at.
John: your creation of the masking pattern is so detailed that the amount of work you put into it is obvious. Eduard used to make 1/72 masks in the A and B patterns, but they seem no longer available; so I've had to resize some thumbnail pictures, then cut out of masking tape. Your method is astonishing in the best sense.
I built up Brian Lane's Mk I for the Battle of Britain group build last year, using a 1/72 Airfix kit and aftermarket decals (from either Poland or the Czech Republic--can't remember which), that took no account of the white "K"--so live and learn, and usually too late. Yours is an extremely attractive model of the real thing.
As far as roundel placement, seems I read somewhere they go at the point where the fuselage is widest (?). Your placement certainly looks fine.
I've been working on my Spitfire(s) in fits and starts; had to wait for paint to come in the mail and now have some. Once again, I've turned to the old Airfix version (which is so simple as to be actually relaxing to build), but have both Tamiya's pricier offering and the latest Airfix rendition that came out last year. I hope to do all three.
Again, great work!