I outlined the scoring system in the Everyman thread a few weeks ago. Basically, use this:-
You get 10 points for each correct answer. There are 28 clue, hence 280 points.
There is a target time of 15 minutes and for every complete minute you are under that time you get 15 bonus points. So, for example, if you finish in 10 minutes you get 280 + 75 (5 x 15) bonus points. But, if you finished in 10 minutes and 1 second you'd only get 60 bonus points because its only 4 complete minutes.
Lastly, there are no points deductions for going over the 15 minute target time so, assuming you're entry is correct, you'd get 280 points whether you finish in 15 minute or 15 hours.