Comment 1 for bug 1556983

conversion has no way of specifying a location for the toc, but you can
use the option "Do not add Table of Contents to book", and insert the
ToC yourself in the source file. Then the resulting MOBI file will have
the ToC wherever you inserted it.

There is a slight wrinkle if you are starting with a DOCX file -- namely
that although the ToC will indeed be where you placed it with the Word
Table of Contents tool, the ToC button on the Kindle will not work. In
order to workaround that, you will need to convert the book to epub, open
it in the calibre editor, and add a guide element in the epub pointing
to the location of the toc, then convert the epub to mobi. The toc
button will then work.

I will look into fixing docx input so that this workaround becomes
unneccessary in the next release of calibre.