Square Layout From The Center Of The Room
Start by snapping a chalk line through the centre of the room (line Y). The next line (X) must be exactly 90 degrees to line Y to form a perfect square corner. To ensure this angle, do the following:
From the centre point (A) of line Y, measure 800mm along line Y and mark that point (B).
From the same centre point, measure 600mm in the general direction of where line X will be and scribe an arc.
Return to the original 800mm mark on line Y and measure 1000mm, scribing an arc that crosses (point C) the 600mm arc you made in the previous step.
Snap a chalk line through the conjunction of the two arcs at point C and the center point of line Y. This will be line X, at an exact 90-degree angle to line Y.