Dave

Hi carol...

It really depends what look you are after with the border....

Are you wanting the border to frame the main tiling field and nothing on the outside of the border, then yes run with the walls....

If you are wanting a picture frame type border and the main field tiles outside the border as well as inside then sqaure the border up and the outside tiles can run off to the walls and then your diamond inset will be true to the border....

Like is said it reall depends on the look you are after.....any chance you can post us a plan of the floor so we can advise which way might look good...?

good luck..