If you know the charge coefficient (i.e d in C/N) you should divide the permittivity of the material to give the voltage coefficient (g in V m/N), which will tell you how much voltage you get per unit stress. does that help ? The product of d x g is a better guide to how much power you can generate, but piezoelectrics are best used for power generation in a resonant mode, so the details are more complex.

Dear Adolfo

The attached file is my paper, may be clarify what you ask about

The basic equation is:

P=e*eo*V/(kt*d33*t)

P=pressure output, e=relative dielectric, eo=permittivity in vacuum, V= voltage applied, kt=coupling factor, d33=piezoelectric constant, t=thickness of piezoelectric layer.

If you want volts out as a function of pressure, rearrange it.