Hi
if i have 2 cubes “A” and “B”
how can i calc the rotation of a imaginary line between A and B, and later apply this number as a rotation constraint in a bone?
Tanks 
I’m not sure if this will help you with what your wanting to do here… but have you looked into using Drivers…
say you have a cube ‘A’ you want to effect cube ‘B’ … one way to do this is to use a ‘driver’…
Select cube ‘A’ and in the Righthand tool bar find ‘transform’ then rotation… > ‘X’
I Right click over ‘X’ axis… and I get a popup menu… in that menu there is ‘Add drivers’ and ‘Add a single Driver’… I select ‘Add a single Driver’ as I want it to only effect the ‘X’ axis…
I then need to open a new view port… Selecting it as a ‘graph editor’… then on the bottom tool bar I will find “f-curv editor” I select that and I get a choice to use ‘Drivers’ … now in that view port I can see my ‘driver’ that I created on that ‘x’ axis… if I hit 'N’Key I get a tool bar on the right hand side that gives me all my information on using that driver…
here’s more information about them here…
http://www.blender.org/development/release-logs/blender-240/ipo-drivers/