Calc a "Prism-decentration" Need help

[Oculus_OD]

In my memory, I remember a formular to calculate a decentration prism, for small diameters in to large spectacle frame.
But I cannot remember the formula...

Can someone help me?

[essegn]

∆ = dec * F

∆ - prism
dec - decentration [cm]
F - power in 180°

F = Fsph + Cyl * sin²θ

F - power in 180°
Fsph - spherical power
Cyl - cylindrical power
θ - cyl. axis