I have a Sony PRS-T1, and I wanted to read manga on it. I use Mangle to reduce the image size and create the CBZ. I then convert the CBZ to EPUB using Calibre, with the option "Disable comic processing". The problem is that Mangle creates images with a 600x800 resolution, but the PRS-T1 needs 600x754. Calibre takes care of this, but then the PNG images file size doubles. So a CBZ file of 100MB after conversion is 200MB.
żIs there a way to enhance PNG compression on Calibre?