Dental braces appear to be deductible, but the comment around 'cosmetic' is vague. If I have a letter of clinical findings for the braces is that enough to claim?

Generally speaking, orthodontic services do qualify as a medical expense. This includes fillings, dentures, implants and other dental work (braces, etc.) not paid by your insurance plan. The only exception is dental work that is purely cosmetic, such as teeth whitening.