For me it varies how much I can write, depends how hard I have to think about it. If i miss something out I generally put an asterisk there because it's usually reasonably long what I want to add in - OK, it might not be the best presentation wise, but at least I'm getting as much as possible down. I always plan my answers, but don't always stick to them (if something better comes to me while I'm writing, I'll put it in) And I always finish my answers, even if it means missing stuff out, because I feel so much better if I actually finish them. (similarly, I never miss a question on an exam, even if I have no idea of the answer, I guess)