@david,
Very few of my calibre epub cover jpgs are 3:4 ratio, but then again neither are the 'thumbnail holes' on Home screen, View Details and Library views (both list and cover). Even if the thumbnails themselves are stretched to 3:4 as they are created, once they are displayed they are restretched to perfectly fit the non-3:4 (more like 3:5 ?) thumbnail holes, i.e. they become less 'distorted' again. The only place where the 3:4 thumbnails are displayed in that ratio is when the biggest one is used as a screensaver. But, for me, the edge-to-edge 'distorted' look is a lesser evil than the horrible white sidebars (
I realise this is a purely personal preference).
Whatever the calibre driver is doing right now works perfectly for me on fw2.1.5 so I hope you don't 'fix it' too much
P.S. Is there a reason why the N3_FULL.parsed thumbnail is created as 600x800 rather than 758x1024, on the KoboGlo?