I found that it was. Really, flanking/not flanking is a binary trigger. That just leaves me to handwave relative distances between points.
Re: larger discussion about per encounter and per day abilities.
I've done some fighting in the past. I'm just guessing, but I can probably hurt someone the most with a kick to the pelvis. For some odd reason, I've never been in a fight where I kicked the other guy in the pelvis 12 times in a row, while never attacking him with my hands. That's what happens if you allow everything as an at will attack. It isn't realistic that you would use only your best possible attack, over and over. The ebb and flow of the fight will force you to use other, less optimal choices. I think its fine to represent this with a selection of at will abilities, some situational abilities, and some abilities you can only use a limited number of times. That's the least cumbersome way I can come up with to mimic something like how often in a fight you can throw a heavy kick instead of a quick jab.