As a Nemeth transcriber you will be using "The Nemeth Braille Code for Mathematics and Science Notation 1972 Revision" as your primary resource. Your secondary resource is "Braille Formats, Principals of Print to Braille". Make sure you have also downloaded any code updates from the BANA website. Rule 13 of Braille Formats covers information on transcribing tests and examinations.
I do know that BANA has a committee currently working on creating guidelines specific to the production of examination material. As with other committee work, these things take time to develop. But rest assured, they are aware that there are questions and concerns regarding the transcription of exams, and are working towards completing their documentation for BANA's approval.