I have read that you can put a strip of crayon down the pipe and the point at which it no longer melts and burns off is the place to put it but I have also read that that isn't accurate either.
I am quite sure race teams like NASCAR that use them do analytical modeling with CAD software then do testing to fine tune. The position is going to vary for every engine/exhaust combo so even a kit from a reputable company is going to be a compromise.
