With regard to the manual the Specular value for the new Principled BSDF node can be computed with following formula:
$\text{specular}=\left(\frac{\text{ior}-1}{\text{ior}+1}\right)^2/0.08$ For example:
water: ior = 1.33, specular = 0.25
glass: ior = 1.5, specular = 0.5
diamond: ior = 2.417, specular = 2.15
But I am getting other numbers. Is the formula wrong? If so, what is the correct one?