Firstly, it really depends on the wording of the question. So the opinions I'm going to give you have that big, fat caveat in front of them!
For example, if the question asks you to evaluate two strengths for 6 marks (for instance), then you need to structure it:
- Strength
- One or more corresponding weaknesses
- Opinion on the strength
- Repeat
But if the question is an overall evaluation, a better structure might be:
- Overall opinion
- Introduce a feature
- Strengths of that feature
- Weaknesses of that feature
- Introduce a second feature
- Repeat...
- Keep doing this until you've written a good length response for the time and line allocation appropriate to it...
- Finish with an overall conclusion
In terms of line allocation, roughly 1.7 mins per mark and roughly 3-5 lines per mark is a good ballpark. In terms of number of points for the number of marks, I don't think it's wise to do one argument per mark for those big 'overall' global questions. You have to balance number of points with detail, but MOST good 8-markers and 10-markers will make more than just 8-10 individual little points.