Regression Analysis ( Curve fitting for hyperelastic model under compression)

Hi

Im currently trying to find out the analytical solutions of few hyperelastic models.

Just need to clarify on the stretch ratio.

My model is incompressible, homogeneous and isotropic.

model is 10x10x10mm being compressed by 50% strain.

For example Neo Hookean S11

S11 = 2 x c10 x ( lam - lam^-2)

How do i calculate the stretch ratio for my model?