A client/partner needs to find the information immediately, but they also need the evidence to know how you came to those conclusions – in case you made a mistake. If this was a task I had in practice, I’d, as concisely as possible, detail the key commercial issues and conclude with my opinion. Then, at the end of the email or as an attachment I’d present a more detailed explanation for each of my claims – but this should still be concise, relevant and clear.
A client is busy. They don’t come to lawyers for a detailed explanation of the law (unless they ask for it). They want to know: Is doing X within the law? Or out of X, Y and Z – what’s the best option? – In which case you need to understand both the law, their business and the industry to form a view of what’s the best action to take.