Hefsek Taharah on Erev Shabbos

A woman performing a hefsek tahara on Friday, and the ruling of the poskim is that ideally, she should perform the בדיקה (examination of herself) before the onset of Shabbat. Does she also need to insert the מוך דחוק before Shabbat begins, or is it sufficient to do so before sunset? The basis of the question is that since the reason of the מוך דחוק is to clarify the beginning of the seven clean days, it should therefore be sufficient to insert the  מוך דחוק before sunset.

Shalom Rav 

It is permissible to insert the מוך  before sunset, even after lighting the candles. 

According to the Halachot of the Gaon, Harav Fried, Shlita