Here I found the explanation

Article Possible new phenomenology in turbostratic graphene bilayer ...

You rotate one lattice point, that can be represented as r=m a1 + n a2, (m,n) to an equivalent position t1=n a1 + m a2 (n,m)(Where a1 and a2 are the lattice vectors of graphene (for instance (Sqrt(3)/2,-0.5)*a and (Sqrt(3)/2,-0.5)*a ) ) , it is called a a commensurate rotation.

It is then not very difficult to calculate the rotational angle between these two vectors r and t1 (using dot product definition), You obtain then an analytical result of the angle between the two vectors.

cos Theta = SomeFunction(n,m)