I would suggest "eyeballing" about half a milligram and starting from there. I do not take benzos often. That being said, a few weeks ago I found half of a 1mg xanax and took it, in combination with the heroin I was doing that day. I did not realize how messed up I was but everybody else did. I am glad I did not do more.
The problem with benzo's is you NEVER feel as f*cked up as you look. You think you're talking fine, walking straight, and you can't understand why the officer is taking you to jail lol!
Be careful man! Take .5mg and see how that works. Then, the next time you dose you will have 2.5mg. If you didn't like the experience on .5 you can up it, but it is extremely dangerous to just "keep taking" xanax on xanax.