Why are you not using kindlegen?
- kindlegen supports Fixed-Format IIRC.
- kindlegen produces dual-MOBI which means people who buy the book will get AZW3 if their device supports it. (There is an option buried in calibre's conversion settings to do the same.)
- kindlegen is blessed by Amazon -- by which I mean, people have reported their files being rejected on the grounds that calibre created them.