Part of writing good, detailed rehearsal reports is understanding what other departments need to know in order to do their jobs. I suggest you dip a toe into as many other areas of backstage work as you can - carpentry, scenic, costumes, lighting, sound, FOH, etc - to gain awareness of how each area must respond to what evolves in the rehearsal room.
Also, consider taking a look at our forum game from a few years back called "
Hell in a Handbag," which focuses on exactly the issue of details in reports.
EDIT: Now that I think on it, good rehearsal reporting also requires a knowledge of the skill levels of your design team. If you've got a very creative and autonomous designer they might need less detail than a student designer with only a few shows under their belt. If you've got a sound designer that loves floor mics when you really need overheads, it helps to know that so you can specify. If you've got someone who tends to get a little too creative about their design choices, you would know to just say "please speak with [director name here] about the new painting needed in II, 4."