Sounds about right, and that's a pretty 'harsh' scenario (5mm wall thickness is really, really thick).

I suspect that any Peachy installation is going to be pretty 'tall'. If the bath is 500mm wide, you'll probably want the Peachy at least 1000mm vertically up. That gives you 1000mm focal length in the centre and 1031mm at the sides - so about half the deviation for double the height. It's been a while since I've looked at optics, but intuition says that we're more concerned about the ratio of deviation to total length (ie 31mm/1000mm = 0.031, compared to 59mm/500mm = 0.118) so that would mean that making it even a little bit taller helps a lot.