Building homemade low inductance capacitor
You need to know the exact dimensions of the plastic material including the exact thickness. You need to know the exact area of the conductive material. Depending on the voltage, the conductive material should not go all the way to the edge of the plastic except at the edge where it is joined. Tape is a questionable choice of conductor. The adhesive will be an additional dielectric that you don't know anything about. It will also interfere with making a good connection at the edge.

Look up the dielectric constant for the chosen dielectric and find an online calculator to calculate the capacitance for one layer.