"Relative to the folder location of the stylesheet? I can certainly try that. In fact the fonts and the sample stylesheet I used are both located in the same folder."
You seem to be getting mixed up between the contents of your harddrive and the contents of the epub file.
If you look in the Sigil display on the left, you see the subdirectories within the epub file itself. As I said earlier, an epub is really a zip archinve. If you rename it to have an extension of .zip, you can treat it like a zip, open it with Windows Explorer, etc.
So, once you have the font inside the epub in the fonts directory, when you want to use that font in the style sheet, well, the Styles and Fonts folder are at the same level - one level below the root, just as the Images folder is.
So, the proper syntax is ../fonts/, I think, in the style sheet - i.e. back one level of directory (..) and then into the fonts directory (/fonts/)