No, I tried exactly what you said. I created two xhtml files, the first with the start of the document and the toc. The second with the main document. I made the "text" entry in the guide section point to the main document with no #start at the end of the href. It still jumped to near the end of the file before. That is why I have given up. I'm never using anchors within paragraphs any more. And I had no anchors added.
If you really have it working all the time, please tell me more exactly how you have managed to do what seems to me to require a miracle! It seems to me that you may just have been lucky, since with some books it works anyway without having two files. When I initially tried three books, one worked, and one jumped too early, one jumped too late.