There are international standards for that. I believe it is even more important to create a good topic list. First, I identify together with you what the future user needs to be able to do, based on the information in the manual. These are the action topics. Second, I check what the user should know in order to perform each action. These are the ‘knowledge topics’. All the topics together make the topic list. Each knowledge topic must be related to an action topic. By doing so, we are sure that we are not putting too little or too much information in our manual.
This applies equally well to other types of technical documentation, such as text for tenders, internal procedures, work instructions, training documentation…