G'day Mate (OP) and welcome to TBN from Downunder.often sight glasses hard to see and don't work well if oil is below min or above max of glass.
Looking at your pic. if I had to guess I would say 90%+ as I think I can see line near top.
Use a bright light and get really close to the glass as this often helps. Also clean outside of glass although yours appears pretty clean already.
If still can't see Cat sell hydro oil dyes which helps when oil is super clear. Sometimes they call it "oil glow"