i don't think there is a formula. If its an identify question you can merely write two or three lines. You should just use the marks as a guideline to how many points to have. 3 marks = 3 main points or things they're looking for
there are 15 marks in the section, and 40 mins to do it in. so it's about 3 mins / mark. and if you have a 6 mark, that's 9 mins, which for me is a page
yeah the first section be concise and to the point. they're not going on length they're seeing if you've fulfilled the requirements. (eg give 2 techniques) but I'm not good at the six mark ones...so I'm not sure, I think you're supposed to write a little more, about a page.