How to avoid "presentism" or "historical transposition" in writing papers?
Follow links on historiography - there are lots out there.  Historiography is both an academic discipline and a name for a text that outlines and contextualises a question with particular focus on how and why and when and where it was approached by whom.

Reading in this area will help you to understand why 'presentism' is an issue in academic writing and how to spot it in your own thinking and writing.

The other advice I can give is to say that if you pay attention to any good academic writing you will see that there are clear indications of what is intended as empirical and what is inferred.  

Proper use of references and citations will help you to avoid accusations of being wholly subjective.  If you want to get across a value statement why not find a quote from someone contemporary to the event whose opinion mirrors your own.  Do not forget to include and consider opinions that contradict your own as well.  You can weigh the merits of an argument voiced by others.  Of course, ultimately, your findings will be influenced by your own background and social context.  Try to be clear and honest with yourself and your audience about when and how this is happening.