formula for firing angle of SCR

Hi,

I'm involved in a project which needs the formula (if any)firing angle of SCR.Can u help me out?

For a 3ph system:

[(3* sqrt(2) )/Pi]*Vline * Cos(alpha)

For single phase systems:

[(2* sqrt(2) )/Pi]*Vphase*cos(alpha)

Where alpha is your firing angle

Hope this is of help.