Planning should not only include the weightage but also the type of previous questions that have come. Like if you see that picture based question is coming from that chapter, have a good look at all pictures and study them. Weightage based on chapter is something vague. Long and basic chapters have basically more weightage . One cannot prepare based on weightage because it will be foolish to leave a topic. You may revise first based on weightage. The highest weightage you revise first , that’s all weightage should be used for. Nothing more. It’s risky if you do assume with weightage other things.