Why I am doing is not modyfing the files from the Kobo, but the ePub itslef, before it gets processed.
But I have been thinking that I could maybe just resize the image to fit the screen ratio.
But the dimension you give are not the same ratio, the screen is 800 x 600 so a ratio of 1.333 should be used for the full page cover, but with the dimensions you are giving me I have a ratio of 1.4. And Every other one has a different one. I will have to do some testing for the correct ratio that could be perfect for all sizes.