I think many people would advocate sticking the dbol at the end rather than the beginning. After several weeks myostatin expression limits gains and at that point people either up doses or swap drugs, etc.
Also I think you may be confusing aromasin with arimidex. The dose for aromasin I believe to be much higher than .5mg daily, my aromasin is 12.5mg/ml and I would dose that 2x per day for 25mg daily.
Arimidex is much more potent and in fact .5mg ED is a bit much, I would opt to begin at .25mg ED or .5mg EOD.
That being said its not so much gyno symptoms you are trying to combat with AI, it is the deleterious effects of elevated estrogen in the male body you are trying to combat, although AI does prevent gyno to an extent.
You should adjust your dosage according to how you feel; you don't want high estrogen but you don't want rock bottom estrogen either. If you feel a libido drop and have some water bloat, your estrogen is too high. If you have a libido loss and dry or achy joints then your estrogen is too low.
If you do experience gyno, take some nolvadex during cycle until it subsides. 25mg/day works well for me.
As far as when to discontinue AI use, if it is arimidex, I would taper off during the two weeks that the test is clearing your system before you begin PCT, and don't use it during PCT. If it is aromasin, I would still taper off it during those two weeks, but if you feel any kind of estrogen rebound during pct, you can take some (12.5mg/day) to help out. Now, expect to feel some kind of drop during pct, but if you libido tanks or you start getting soft and watery real fast then you can have some.
The reason behind not using one over the other is in their mechanisms of action, and this is simply my understanding I could be incorrect. Aromasin kills the aromatase enzyme itself, so the levels of it available are reduced until you build more. Arimidex interferes with the aromatization at the site, so it blocks T to E conversion, but the enzyme is still floating around, potentially at a higher level. So while arimidex is more effective, its better for consistent use whereas aromasin can be used intermittently. If I'm wrong someone feel free to correct me.