The liquid crystalline model biomembrane system consisting of a stack of interacting membranes is studied in the realistic soft confinement regime by the newly developed Fourier Monte Carlo simulation technique. In this regime experiment and simulations show that the functional form of the fluctuation pressure is more nearly exponential rather than the power law valid for the hard confinement regime. The simulations provide quantitative improvement over perturbation theory. It is shown that the harmonic theory that is routinely used to interpret x-ray scattering line shapes is valid.