Here is a link to ECMA-119 which is the equivalent to ISO-9660.
As you will see in the PDF, each directory and file name has several bytes of overhead. So, you will not be able to simply count all the letters that make up your directory and file names. It may seem a little overwhelming at first, but once you get a basic understanding of how it all works it's not so bad.
Remember that every file ends on a sector boundary even if most of the last sector is all zeroes (unused space).
Another thing to understand is that no data is stored on the disc before 00:02:16. This is where the ISO file system starts being defined. The first sector (00:02:16) is the PVD (primary volume descriptor). You can use that info as a reference while reading the PDF to see what is stored there and what comes next. Good luck.