Exactly how much hair is “uncovered”, Rav Falk, who by many accounts follows the most stringent opinion at all times, writes the following….
“Hair which is difficult to contain in a regular well-fitted hair covering is halachically (according to law) exempt from this obligation. This refers to hair which grows on the temples next to the ear or on an exceptionally low hairline that extends below what a net or tiechel (scarf) would normally contain.”
The quote comes from Modesty- An Adornment for Life, Rabbi Pesach Eliyahu Falk (exact page number is somewhere in the 230s I believe, dont remember exactly where).
He continues….
“Although there is no obligation to cover such hair, nevertheless, if local shomrei mitzvos (observant Jews) are stringent and cover them, the halacha (law) obliges women who live in this locality to behave likewise.”
On the flip side…There is the opinion of Rav Moshe Z’l in E”H 1:58 (I dont do hebrew fonts) that seems to permit a woman to show one square tefach of hair. However, he adds that since a woman’s head, on average, is about two tefachim wide, she should not allow more than half of a tefach to stick out.
The question, I guess, boils down to who or what established/establishes minhag hamakom? Is minhag hamakom like Rav Moshe, or like the Poskim Rav Falk relies on (I dont remember who he quotes).