Wow, three questions in one. First of all, there is no ideal body fat that you should as long as you are below an unhealthy amount, which is generally (depending on the individual) under 32% for females and less than 24% for males. How low you desire your body fat % below the acceptable healthy range is personal based on how you want to look.
On the first part of your question regarding the consideration of women’s breasts, the answer is yes they are figured into what is an acceptable healthy range for women. In other words, as mentioned above, women’s bodies, because of breasts and other parts requiring extra fatty tissue (e.g. around hips) in order to support maternal functions, can carry more total body fat before it reaches an unhealthy range. Therefore ,to the question about having little to no breasts, the rest of the answer is that if a woman with small breasts estimated their body fat through the use of skin calipers or any other method that uses gender formulas, the body fat final number will be slightly overestimated because skin fold formulas use averages.
One final note: when it comes to positively changing body composition, at the end of the day, no one cares how accurate the body fat measurement may be. As long as you use the same method and tool each time (preferably the skin–fold calipers because it’s practical and consistently accurate when repeating measurements), you should only care that the given number is moving in the direction you are working toward. In other words, the measurement tool needs only to be directionally correct.