QUOTE FROM INSIGHTS-
>>It is not clear, however, why this will suffice. According to Rebbi Chanina, it ought to be necessary to check specifically on the first day in order to establish a Chazakah that she is clean for the next six days. The Dagul Merevavah should have said that the first examination must be on the first day, while the second examination may be on any other of the seven days -M. Kornfeld.)<<
One could say the initial "chazakah" is established by the hefsek tahara
Yes, I think that you are correct -- the Hefsek Taharah can certainly establish a Chazakah. However, I think that the point I made about the Dagul Merevavah's ruling is still valid, although I should not have used the word "Chazakah." What I meant to say was that according to Rebbi Chaninah, the Zavah's count must conform to what the Gemara calls "Sefurim l'Faneinu." The Gemara elaborates that according to Rav, who argues with Rebbi Chanina, it is obviously not necessary to have "Sefurim l'Faneinu" since he holds that if there was a proper Hefsek Taharah, the first Bedikah of the 7 days may be on the seventh day.
From this statement it can be learned that even though there was a Hefsek Taharah, it is not considered to be Sefurim l'Faneinu unless the woman does her first Bedikah at the beginning of the 7-day count. That is why I thought that the Dagul Merevavah ought to have required a Bedikah on the first day, besides the Hefsek Taharah on the previous evening, according to Rebbi Chanina who requires "Sefurim l'Faneinu."
Please continue to keep us on our toes!
-Rabbi Mordecai Kornfeld