I will explain the license grading system in a second, but the main rule of thumb is you need to compete at Regionals or Nationals to gain any license above F5. So if you haven't done either of theses series, then you'll be an F5.
There are various mathematics to working out license grades, but in general;
At the end of a Regional series the top 20% of qualifying drivers gain an F2, the next 30% will be F3, the next 30% are F4 and the remaining stay F5.
At Nationals, the top 33% gain F1 and the next 17% are F2. No other license grade is awarded at Nationals.
That is a rough guide and the full details can be found in the BRCA handbook, page 24(ish)
http://brca.org/BRCA/library/BRCA2009.pdf