If a college guarantees to give need based aid (Franklin and Marshall or the University of Richmond for example), would the npc be 100% accurate assuming all information was put in correctly? If I need aid and they guarantee it need based, would it be safe to apply ED based on the npc?
No most NPC are just an estimation of your financial aid.
It is only going to be as correct as the information that you put into the NPC. There are situations where the NCP can be inaccurate. For example:
Are your parents married to each other?
If not, did you include your information from your non-custodial parent? (these schools request information from both parents whether or not they are married to each other?
If your parents are not married to each other, are they married to other people? If you have stepparents, their information must also be included.
Do either one of your parents own a business or are self employed?
Do either or your parents own property outside of your primary residence?
They are sometimes not accurate for international students.