There are so many factors to consider that it's barely worth trying that hard. You can't be sure of grade boundaries, others getting answers right, remembering your own answers and so much other stuff that the margin for error is significantly greater than grade boundaries.
If you really must, I'd suggest looking at previous mark schemes for similar style questions. The longest questions I've had to deal with are 7 marks and most are under 5 so it's easy to just give yourself half or a bit more. Because of ECF, it's still possible to pick up the majority of marks even if you don't get the correct final answer.