I would say divide the total number of pages in the booklet among the questions, so it usually comes to around 2 pages for 10 marks. If we follow by this, atleast 3-4 pages for 20 marks and so on.
Ayyo... I haven't gone through the paper. But I hope the coming exams would be better for you. Try not to skip any topic as much as possible. Try to pick 3 units which feel relatively easy and cover everything in them. Study strategically is all I can say. Best wishes