When you use the actual numbers of people, you get those numbers by using the base rate: 10,000 women total, of which 100 have cancer (that's the base rate in action), of which 80 test positive, etc. So if you use the numbers 80 (= 0.8 * 0.01 * 10000) and 950 = (0.096 * 0.99 * 10000), you're not ignoring the base rate. You would be ignoring the base rate if you used the numbers 8000 and 960 (80% and 9.6% of the population of 10,000, respectively), but those numbers don't refer to any relevant groups of people.