PCB Layer Stack Dielectric Constant Math
Assuming the frequency is so low that the capacitance as whole board size component is meaningful you can well calculate the capasitance per unit area between the two remaining metal layers with the following formula:

$$\frac{C}{A} = \frac{1}{\frac{s_1}{e_1} + \frac{s_2}{e_2}}$$

Where e1 and e2 are absolute permittivities of the dielectric layers, s1 and s2 are their thicknesses. This applies the idea to have 2 capacitors in series.