It works perfect Vrul, THX!
My project is complete now, this was the last thing I needed to finish it.
The project consists on an horizontal inversion of Blizzard's FocusFrame, to make it symmetrical with the TargetFrame on UI's that use Target on left and Focus on right (very common frame distribution on high level PvP).
And this is what I really wanted to do (imitate the castbar positioning of the TargetFrame on the inverted FocusFrame, which I can do now using your formula and the FocusFrameManaBar:GetRight() value). I used just the default focus in the question to make it more simple without having to explain the project.
I will release it tomorrow, now I need to sleep.
THX 