If you haven't already, I'd look for as many photographic references for that mark of the Spitfire as I could find. If nothing else, I start with Squadron's "In Action" and "Walkaround" booklets. Then any hardcopy references. And then online images are next, though with the caution that pictures of restored warbirds are moderately valuable references, because you don't know what compromises or changes the restoring staff might have made. This probably doesn't help you immediately, but I can't tell whether those details are accurate, or if the instructions are. I'd have to look 'em up.
Maybe others here in the Herd have some good references to help, though.