I believe I was about 30-40cm away from the subject. You have to watch the corners really carefully when shooting. What I usually do is to do a quick check at each corner before I fire the shutter. This is a habit that has stuck with me for many, many years now when I got my first ever 100% VF camera, the Nikon F3.
When using flash, I have the wide angle unit put over the FL-50, followed by the Stofen (goes wider) and then the Lightsphere PJ II. Of course, the power will be more limited.
I did not use any lens correction in any of the workflow. When I'm free, maybe start I'll relook at it.