options question

so far in the past year questions, Pa is the discounted value PV of future cashflow and the way to get PV of future cashflow is the sum of Pe and expected NP. therefore my questions are 1. is there any questions that the way we get Pa or Pe is different from the way i have described above ? because using my revision kit from my school, all of Pa and Pe are derived as i have described above and also in past year questions as well. 2. if question the way i have described above to get Pa and Pe are wrong, then is there a general rule to get Pa and Pe ??