The Fresnel equations are derived from considering the three light rays at the interface and requiring that the electric and magnetic fields of the light be continuous at the interface:
The working is basically straightforward, though tedious, and you'll find it done on a thousand web sites so I won't reproduce it here.
However this assumes the dielectric is semi-infinite so we only need consider the three light rays when matching up the electric and magnetic fields. If we have a sheet of finite thickness we now have to include the light rays reflected off the bottom surface of the sheet:
This is a lot more complicated because:
we have to simultaneously match the electric and magnetic fields at both the top and bottom interfaces
we have a more light rays to consider at the top interface
the thickness of the sheet produces a phase shift that has to be included in the calculation
Presumably there is a closed form equation giving the reflected intensity at the top sheet, though I have never seen one. In practice when dealing with more than a simple interface things rapidly get so unwieldy that we usually solve the problem on a computer. In fact I did precisely this as part of my PhD though I was dealing with multiple sheets not just a single sheet. The method I used is described in Optical Properties of Thin Solid Films by O. S. Heavens, Butterworths Scientific Publications, London 1955. It is on Google Books, though sadly not a scan.
So the answer to your question is that there isn't a simple answer to your question. You can't treat the surface and the thickness of the sheet separately. You have to do the full (complicated) calculation to find out what the reflectivities of the two polarisations are.