Originally Posted by st_albert
FWIW, here's how I do it (for an epub intended for conversion to mobi):
In Sigil (0.3.4 if it matters) I mark the file containing the first chapter with the semantic "text". Just to be clear, right-click the file in the book browser, choose "add semantics" and click "text".
Make whatever other changes necessary for the mobi conversion, such as adding an inline TOC.xhtml, removing the cover page (but not the image) and so on. Save and exit.
Then, expand the epub and edit content.opf. Make whatever changes are necessary to the guide (e.g. regarding the cover), and change the title= field in the <reference type="text" guide entry to title="Start Reading" ...
Save, re-pack the epub, and use kindlegen to convert it to mobi.
The kindle will open the book to the page pointed to by the "text" guide entry. The kindle's menu | go to | beginning entry will also take you there.
Oh, and BTW if I'm not mistaken, during conversion Calibre does not pass on to mobi those <guide> elements that are non-standard or not supported by the mobi format. At least, that used to be the case several versions ago. I have not tried it lately. Bottom line is that the "text" guide element was ignored.
Actually, for any epub->mobi conversion, merely marking the "text" semantic works, as I posted originally. What has not worked for the OP is that s/he is using Calibre, which apparently is ignoring the "text" semantic that, I agree, works perfectly (actually, without all the fooferah) in a straight-up KindleGen convo.
I've never needed to edit the OPF and change the guide item; "text" works just fine for anything through MobiGen, KindleGen or Previewer. But, alas--it seems NOT to work via Calibre, but as I don't use Calibre as a production tool, I have no answer to this other than exploding the epub and adding the start location manually.