[Chimera-users] rigid body rotation of subunit

Hi I have the structure of two homologous protein. These proteins are dimeric. when I am superimposing chain A of one protein on to the chain A of other, it looks like chain B of one protein is related to chain B of other protein by rigid body rotation. How can I calculate the angle of rotation in chimera? Is there any way, I can draw a vector along beta sheet? Thank you. Dhiraj