Get custom field from page, in a post?

Question

I'm not quite sure how it works, if you're on a post (single.php), does it count page.php as the page it's on?

What i need to do, is get a custom field assigned to a page, from a blog post. I need it because i want the user to be able to change the "Read more" text in the post excerpt.

I could get the custom field by just using a static page id, but if the user decides to change things up the id might change, i need it to be dynamic.

What would be the best way to go about this?