@Sabaot - there are several threads in this forum about converting from Word.
If you you have tabs or empty paragraphs in RTF files you will get unsatisfactory results. Get rid of the tabs and blank paragraphs by using Paragraph formatting for things like indents and paragraph spacing etc. And use the heading styles that come with Word for Headings.
If you have Word 2007/10/13 I recommend saving the document as DOCX and using the DOCX conversion feature that's been recently added to the core Calibre code.
DOCX files are typically 90% smaller than the equivalent RTF - ie a 150KB RTF will be more like a 15KB DOCX, the conversion (to EPUB at least) is much faster and the output (to EPUB at least) is very faithful to the DOCX file as seen in Word, and the EPUB is amenable to editing via Tweak Books or Sigil - which IMO is not true with conversions from RTF, there were so many MS artefacts that the text was almost impossible to find.
I'm pretty sure you will see similar advantages if you convert the DOCX to AZW3
The key thing is get rid of the tabs and empty paragraphs and use Words Styling facilities and if you can create DOCX's then I suggest you do that. I'm in the process of converting 20,000+ RTF's to DOCX and recreating the 20,000+ EPUBS.