Hi Paul....it would be very difficult now to work the bottom of the dish, but if it's still possible to attach the underside of the dish to the chuck that you used, you can remove thickness from the top down. What I did was to sweep across the bowl with a bowl-gouge, towards the centre, but (and this is the important bit) without removing any material from the corners......so your gouge starts cutting further in towards the centre, which reduces the thickness of the edge without removing any more from the corners.
My best advise would be to re-mount it on a chuck and spin it by hand, slowly, whilst moving a gouge across it, you'll soon see where the gouge will cut.
Keep those fingers out of the way, though, and best of luck!
Les