Hey connormurry-
Rather than multiplying by a static number (such as 1920 or 1080) you are multiplying by the actual viewport size. If the size of the viewport changes it would also change these numbers. So the difference between fullscreen and in editor would be expected. If you change the size of your editor viewport and ran it you would again see different numbers. Connecting a print node after the BreakVector2D will let you see what the resolution of the current viewport is.
Cheers