Comment by msephton

You're quite right, the screen would be centred with margin/border around it.

Whilst the CRT is 9", according to period repair guides the screen should be adjusted so that the visible image was 7.11" x 4.75", pretty much exactly 1.5:1. This meant 72dpi, which was to match PostScript point size for print output and WYSIWYG.

So it's your 8.55" diagonal.

Some classic Macintosh users today are unaware of this screen size reasoning, or don't agree with it, and stretch the screen to fill the whole CRT. Yikes!

BTW, I posted pretty much the same info earlier today at https://news.ycombinator.com/item?id=44105531 — what synchronicity!