To find the centre of a room. Regard it as a uniform rectangle. Ignoring any recesses and projections such as fireplaces and hearths. Measure the full length of each wall and mark the centre points.
Rub a length of string wi th chal k and secure it with nails at the two centre points of two opposite walls. Keeping the string taut. Snap it sharply to leave a chalk line against the floor. Mark a second chalk line between the two centre points of the other two walls: where the two lines intersect is the centre of the floor area.
If the room is L-shaped. Treat it as two imaginary rectangles, regarding the centre of the larger as being the centre of the whole floor area, and snap the chalk lines accordingly.