Take a long piece of sewing thread and tape one end to the inside of the big chainring, at the front of the CR. Now pull the thread back towards the FW/CASS. Keeping the thread taught, carefully sight down the thread and move it until it just makes contact with the back part of the CR. The thread should be just to the right of the center cog on a FW/CASS with an odd number of cogs, or just to the left of a cog on an even numbered one. Change the BB spindle legnth or add spacers to the cup to correct the chainline.