***Prescription note and change:*** In order to comply with the law and meet you presciption needs you must: see the doctor, have the doctor state the minimum level of tint and date the document.  It has a 2 year expiration and must be "refilled" so to speak every 2 years or it becomes invalid.
An example of what a prescription needs to have:
Patient xyz has a medical need to have no darker than 20% light transmitting film applied to his automotive windows.  Date: 08/1/2008  

It must have the typical prescription credentials on the note.

50% is a light tint and state legal.  5% is a limo tint. So your range is probably between 50% and 3%.  Medium is approx 35%-20%.  Dark is approx 20-10%. Extremely dark is 10%-2%.  Below 2 is more like opaque.