Self Storage - Equilibrium Sq Ft per person in rural areas

[Steve Cheslock]

I have read that for determining equilibrium a good rule of thumb is to use an avg of somewhere between 7-8 sq ft of self storage per person within the radius you are analyzing.  However we all know each market is very different.  

In analyzing small towns in more rural areas what number do people suggest using (or do they use when doing their analyzation)?  

Thanks in advance.