XPS measures 100 Å (10 nm) of material on or in the surface. XPS quantification totals to 100 atom%
As a rough estimate 1 atom% of an element amounts to ~1 Å of that element . Ten (10) atom% equals ~ 10 Å of one element.
If however, you want to estimate the thickness of a chemical compound, such as PDMS (silicone oil), you need to consider that one polymer unit of PDMS has 2 carbons, 1 silicon and 1 oxygen (-C2SiO-). As a result if you find 4 atom% of Si from the survey, we can estimate the thickness of the PDSM as ~16 Å .
This results from 4 atom% Si x 4 atoms (-C2SiO-) or 4 atoms x 4 Å = ~16 Å of PDMS.
For SiO2 on Gold, we would estimate 7 atom% of Si to be bound to 14 atom% of Oxygen. The total amount is ~21 atom% of SiO2 on Gold. Roughly estimated as 21 Å of SiO2 on Gold.
There are much more sophisticated calculation to measure thickness of overlays. This method is just a rough Rule-of-Thumb method which estimates thickness.