I have some comments that apply if you plan to publish this book on Amazon. (If you are not intending to do that then you can disregard the following.)
I strongly suggest that you use
Kindle Previewer to convert your book instead of calibre. MOBI files created using calibre are sometimes rejected by Amazon and do not include the information needed to allow the Enhanced Typesetting features of the Kindle platform to be enabled for your book.
There isn't much point in worrying about the start position for your book. Amazon will override whatever you set as part of the publishing process anyway.