Diameter of an 11' foot high, 3" (2&7/8) steel post footer hole

I know that the general standard for post footers is to make the diameter of the footer hole 3 times wider than the post itself. However, I want to make sure that even in an 11′ foot post (which seems kind of high to me), that this rule is still ok. Or should the footer hole be a bit wider for such a high post? The questions regards the corner post of an 11′ foot high chainlink fence. Thanks in advance!