Cover thumb cockeyed in Bookshelf view

[clmweb]

Hi there! I've lurked here a little while, and finally registered so I could ask this question.

Okay, so I'm finishing up the creation of my first Epub book, using Sigil. For the most part, it works fine, displaying how I want, including some internal images for chapter heads, flourishes, etc. I'm viewing the draft versions on my android phone, using Mantano reader and Alkido reader (both of which tend to display other epubs just fine).

I've seen conflicting info on the best size for ebook covers, and have gone with the standard 800x600, 72dpi (but during my trouble shooting of this problem, tried several other suggested sizes/resolutions to no avail.) I was having embedded cover aspect ratio issues until I found this thread. I used JSwolf's code snippet (swapping out the correct location of my cover, natch) and while the cover doesn't fill the screen side to side, it's big enough and not stretched funny.

BUT (and now we're coming to my main problem) on the bookshelf view of the reader, where I've got all the thumbs of the covers, my cover is (if I use the 800x600) ... slightly off center, showing white along the left and top margins, and cut off ~15% along the right and bottom edges. If I try other sizes, it's alternately too small all together (and appears squished within the thumbnail frame) or even more off center.

And I can't for the life of me figure out what I'm doing wrong. I've done add semantics>cover image, I've tried having them be two different files (one for semantics, one for the internal cover page), I've tried having them be 1000px on the longest side, and then bigger than that, and then considerably smaller.

And I have this issue in both Alkido and Mantano (my go to apps for reading epubs). And (being someone who tends to learn quickly when teaching herself new programs/etc), I get the feeling it's one stupid little box unchecked somewhere, or line of code slightly off or something, but it's the one thing holding me back from finalizing this darn thing, running it through some validators, and delivering it so I can get my $.

Anyone out there have any thoughts?