This will seem basic but also a situation that were faced with…

Understand that on a quote/quote line group/quote line, the Subscription Term field will override End Date. If you use end date, by all means do so, but don’t feel like adding a term will make a difference.

Also look for fields called Effective End Date in some of the objects. It’s a formula field that does a nice job of saying what the end date should be and so how the proration multiplier is set.