Working back towards door, cut closest end of quad square. Measure length again and add 2mm, to spring quad in place and keep joint closed. This time, cut a 45° angle, using mitre box.
Use coping saw to cut along contour of curve so fine blade is at 90° to back of quad.
The result should fit snugly over curve of quad already on floor when at right angles. Refine cut with a file, if necessary. Spring in place and nail to floor. Continue working back towards door.