Dear Aspirant, there is a quota of seats for all Pakistan "merit" or consider it as open merit. say there are 7 seats reserved for "Merit" then first 7 position holders will get allocated on this quota.
Secondly about setting target percentage , well that is actually relative to every CE , sometimes it is high scoring and at times over all it is a low scoring result.
In any case you have to be at par if not above with the avg. written marks. 75% in written means you are among the toppers. On avg. if you get around 62%-68% you will have a pretty good chance to land your desired group provided that you score good in the interviews.
Women quota is 10% of seats available for any group. e.g. if a group has 10 seats then 1 seat will be reserved for women, irrespective of the fact that if other 9 seats get occupied by women on merit.

For the written exams for now , work hard and forget about the maths and the probabilities. Your job is to study and leave the probabilities to Allah.