From my understanding of the text, the actual maths behind determining the Roll centre height and roll steer are complicated (They do not go into the equations in the book).
What is presented is a simplified approximation of Roll Centre height and roll axis (Roll steer).
This is done by the roll centre as discussed already (front mount and upper rear mount, vertically above the axle) and the "swing arm" which is derived from the slope of the front 3/8ths of the spring (Q to R), transposed to the roll centre (C to O). The slope of this line then determines roll steer and the end point (C) is the Instant Centre (length C to O = the 3/8 spring length L).
In the text SVSA (Side View Swing Arm) or the single point at which the suspension will rotate around.
I hope that helps.