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How do i revise?!?!

I'm currently in year 11 and my exams are in 3 months. I have made a revision timetable with 3 hours per week on the most important subjects going down to 1/2 an hour on less important subjects that I am hitting my target grade on. In my mocks I got:
History - 3
Spanish - 1
Further Maths - U
Maths - 7
English language - 8
Physics - 5
Biology - 5
Chemistry - 5
Computer Science - 6
My target grades are 8's for everything and I have no idea how to get there. Please can someone give me ideas on what to do?! I am trying to do a mind map on each chapter in my revision guide, then flashcards on important points, and a test paper for each subject every three weeks.Oh, and in my maths I am working my way through the Maths Watch DVD. I haven't done any revision so far (not even for my mocks) I am only starting this month.Is this okay or will I just be wasting time when I should be doing something else?