The standard monthly cycle of 28 days would have her ovulating on cycle day 14. Sperm can live inside up to 5 days, so sex on cycle day 6 is pretty safe. Women do sometimes get pregnant from sex on the last day of their period, so there are no guarantees. The fact that you pulled out is an added safety. I wonder if you thought to urinate between your two ventures to make sure there were no sperm left in the urethra.
If you want a number, let's say there is a 2% chance. I guess you know about PlanB and decided the risk was too small to justify that expense.
My doctor says the fertile window starts on day 9. By my math, you guys did this on cycle day 8? But you can get pregnant at any time during a woman's cycle. (the way a cycle works is day one of her period is cycle day 1 and you count from there). Pulling out doesn't protect you from pregnancy at all but is better than ejaculating inside of her. Agree that you probably don't need to use plan B but do buy some condoms. This was a close call in my opinion.