I really do not think thar formestane is anywhere near on par with arimidex. Personally I would use a low dose arimidex on cycle. However, what so many people do is ge obsessed with low estrogen during a cycle and go nuts with AI's losing a good deal of the effect that the hormone was going to provide.
In fact my first cycle ever was 100mg ED of test prop for 6 weeks, however for some reason I thought that I was supposed to use letrozole at 2.5mg EOD. Well I did and I literally got next to nothing out of the cycle. Literallyu thought the gear was bunk and had it tested. Low and behold it was not and I therefore believe that estrogen plays a crucial role in the gains we get from steroids we need it in our bodies to an extent we just dont want tooo much.
I personally would not use an AI until I was using more than 750mg of test a week. And even then it would be at a very low dose so that I still had about 50% circulaing estrogen, like .5mg Arimidex EOD.
I would ditch the formestane and arimidex. Maybe because it is so weak you can use it but why? Also Epi is said to help with estrogen so hell run that as your anti-e.
Good Luck!