In general...
 - Use a "Before" trigger when you want to make additional changes to the records the trigger is acting on
 - Use an "After" trigger for everything else
   - when you want to make changes to SObjects that _aren't_ the one that the trigger is on (i.e. if you want to make a change to the `Account` that an `Opportunity` is related to, in a trigger on `Opportunity`) 
   - if you need to change records that are not the ones going through the
   trigger (e.g. you want to make a change to the Account specified in
   the `ParentId` field in a trigger on `Account`)
   - some other scenario I'm probably forgetting

So yes, a trigger set to run `before insert` (and possibly `before update`) would be appropriate here.

That said, a simple calculation like this is a perfect candidate for a __before-save, record-triggered flow__ instead. Flows are generally quicker to develop, and don't require unit tests<sup>1</sup> to deploy.

Do note that since the page to add OpportunityLineItems to an Opportunity is provided by Salesforce out-of-the-box, you won't be able to modify it (unless you override that button), and the calculated discount will not show up on that page (as it's only calculated _after_ you hit one of the "save" buttons).

<sup>1: Not _required_, but probably still a good idea. Hhaving unit tests, and running them routinely, can help detect issues that other metadata changes can introduce. It can help detect them before _your users_ do, which is preferable to having your users inform you.</sup>