I recently reviewed an article for a colleague at Colorado School of Mines, and our advice may sound redundant but it reinforces certain points that I think will help as you think about teaching in the margins. The specific advice offered to geoscientists includes these five key points about commenting.
As a rule of thumb, include a summary comment with no more than two or three strengths and two or three areas for improvement.
Focus on global issues, not minutia.
Marginal comments should be brief, few, and should substantiate the summary comment.
Communicate your expectations and stick to them.
Use rubrics to save time and increase assignment efficiency.
Jon A. Leydens and Paul Santi, "Optimizing Faculty Use of Writing as a Learning Tool in Geoscience Education," The Journal of Geoscience Education.