What I do is try to race through the whole paper. (i.e. skipping stuff that I don't know) Then the left over time, I try to do what I didn't do(no more than 12-15 min when trying to work out question 6 and 7). If I've done all that I'd rather check what I've done a pick up a few marks in silly mistakes rather than waste my time on questions 6 and 7