Been working here 2 and a half nights now. This area is about 125 s/y or so. I chalked a line down the center of the room. On a herringbone pattern there's two points where the tiles/planks meet. The CENTER of those two points has to be aligned along the chalk line drawn down the center of the room. Follow? So depending on the size of the material-----in this case 18" x 36" carpet tile you dry lay some flooring up the center line in the herringbone pattern to ascertain the correct measure from those two points to the center line. Still following? Then you snap those other two lines and you have the layout centered in ONE direction. To get the layout to the perpendicular walls if it's a smaller size area just dry fit. In a bigger space you dry fit enough flooring to get a measurement you can then multiply and find out the best size cuts for that particular material in that particular space. It's not east to do. It's even harder to explain. Maybe pictures will help.