Obin, check my earlier post:
http://forums.pelicanparts.com/showthread.php?threadid=55804&highlight=sensor
As I remember, you don't have to be dead on since they are magnetic pick-up coils. Since you don't have the back of your motor exposed (I was doing my clutch at the time), I'm assuming you're using some kind of depth gage, probe or caliper end to measure the distance? Remember, one sensor counts the teeth of the flywheel; the other one (forward I think) looks at a set screw that zips by for TDC.
Good luck,
Scott