Nick,
don,t worry you are NOT going wrong, its the diagram
Rather than take the measurement from the side face of RS15 to the center of the SHOCK absorber bracket, its best to use the 97mm from the base line & 48mm from the side line of the plate.....
don,t use RS15 as a guide, as you have found its quite easy to come up with conflicting measurements even tho in the end it does work out, but I would suggest that anyone doing this for the first time is to ignore the RS15 part.
The shock bracket does not touch RS15 when in the correct position, the bottom cad picture of page 173 you,ll just see that there is a small gap between the two parts .
also, don,t miss that the RS15 part sits 3mm lower from the TOP edge of the plate as this is the position that part CP6 then sits flush against the end of RS15 & creates the "box " effect that you are making out of all the plate parts.
The bottom edge of the diagram once turned over ( when the box is made ) sits on top of part SB2...........
I,ll explain ...
CP3 is 150mm .....RS15 is 122mm....3mm gap for part CP6.....and the 25mm of part SB2 = 150mm.
The diagram is a little misleading until you actually make the make the parts, but this also may help...........the dimension sketch from page 173 ( as in your post above ) is in fact for the NEARside of the car but needs to be turned around so that the bottom edge becomes the top edge when placing on top of the SB2 part..........this is where the diagram is confusing
I hope this helps, its not easy to write down & explain.
cheers
andy