I agree that it would be best to use the outside dimension and cut to exact length. If the miter is accurate, that would make the final outside dimension work just right. If you use the inside dimension, the you'd have to add the exact width of wood, which means accurate planing of slightly over sized stock. In the photo, very careful measurment shows the lighter piece appears to be slightly thinner than the dark piece. You can see the effect of that on the inside of the miter joint. The outer points of the miter align, but the inside joint has a slight misalignment.
The more I do, the less I accomplish.