I would take 1 mg EOD. I am not sure if this is accurate at all, but here goes.
Some pills don't have the medicine uniformly distributed in them, but rather "injected" into the binding compound of the pill somewhere in the center. NOW I dont know at all if this ist the case with arimidex, but if it is, you would essentially be doing 1 mg EOD if you took a half a pill a day .. so why not just a whole pill EOD ...
Also, if you split a pill, you sometimes lose a few loose fragments, so why not just take 1 EOD if it works and avoid this?
Hope that helps bro, have fun with it whatever you decide to do
NFG