When scribing a board to another object, you can use many tools to mark your cut accurately. Against a straight wall, where your board may not be a consistent width, you can align the board to be cut directly on top of the prior adjoining board, then slide a scrap (of the same width material) along the wall to get a precise mark for your cut.
You can also use a wide washer to get linear and nonlinear marks for your cut. When scribing against objects like stone, you may be able to use the above options for a rough cut, but final detailing usually requires a compass or scribing tool. You also need to account for necessary expansion space in situations where temp and RH will fluctuate.
By Ben Totta