If the unsquare wall is straight you should scribe a line, and use a router with a kitchen jig. If the wall is uneven then you should scribe the line and cut slowly and carefully with a jigsaw. Use a blade which cuts on the down stroke to prevent chipping the surface laminate. With care you should be able to match the wall within a millimetre. If you are tiling the splashback you will be able to conceal the gap.