Calculate rotation angle for two side of cube ( like dice ) from Quaternion.identity

Hi, I want to know how I can calculate rotation angle from Quaternion.identity to specific two side ( Top and Front side )

For instance I want to select two top side and front side from inspector my code is this but If I select first position of top side always front is same as first position of that but if I change top side for example in dice ( 2 is top 5 is bottom and front is 3 ), it works.

I hope I could explain my problem clearly. Sorry for my bad English.

 float angle = Quaternion.Angle (Quaternion.identity, currentFrontRotation * currentTopRotation);
 newRotation = Quaternion.AngleAxis (angle, Vector3.zero) * currentTopRotation;