I was just wondering how you would work out your mark as a percentage, from your mark in a 'negatively' marked test

Maybe if I gave a few details, there were 30 questions, one point given for each right answer, 1/3 points taken away for every wrong answer, no marks given or taken away for any question unanswered,
So a maximum of 30, and a minimum of -10,
Say I answered 16 correctly, and 2 incorrectly, that would give me a total of 13 and 1/3, how would I work that out as a percentage?