All my letter from a dentist said was that my dentist recommended me for a place as a dental student, that he had know my family for X number of years, and that he thought I would make a good addition to the dental profession. My dentist did not document my shadowing hours in the letter, note my academic performance, extracurriculars, or achievements, or anything else really. He just personally recommended me as a suitable future member of the profession and a decent person. Although I think the more detailed the letter the better, the purpose of the letter is just to verify that you are a suitable addition to the elite professional world of dentistry; that you are suitable to be a dentist and will likely succeed as a dental student and dentist.