just find out the Back Vertex Power of the Lens with the help of Focimeter & ADD the surface powers together to find the THIN LENS POWER.
NOW ,The Ratio of the thin lens power to the true back vertex power is the same value as the CVF ( CURVE VARIATION FACTOR ) of the Material of the LENS.
where Ft = true back vertex power of the lens
Flm = sum of the both surface powers ( Thin Lens Power )
nLM = refractive index for which the lens measure has been scaled
nT = REFRACTIVE INDEX OF THE MATERIAL OF THE LENS .