Alpha Helix (Rotational Angle) Formula

Equation for calculate alpha helix (rotational angle) is,

$3\times \cos \left(\Omega \right)=1-4\times \cos {\left(\frac{\varphi +\psi }{2}\right)}^2$

where,
ϕ = Dihedral angle of one residue (angle between two planes),
ψ = Dihedral angle of the next residue (angle between two planes) ,
Ω = The the rotation angle