Arimidex has a half-life of 3 days. So in essence you could take one tab(1mg) every 3 days. You mentioned you wanted to take .5mg's EOD, well with a half-life of 3 days the amount of arimidex in your system on the 3rd day should be around .5mg's.
If you're using longer esters (ie: 2 pins/week) you can take your arimidex, 1mg tab, with each pin and this should have you roughly around your goal of .5mg's EOD through the week.
I use arimidex with my high dose cycles, 1g to 2g of gear per week, at 1mg EOD and if estrogen influenced sides start showing up I then start taking the arimidex at 1mg/day and this always seems to keep sides at bay.