When asking for technical help at the very least let us know the version of firmware you are running.Cliff's Calibration (Mapping) Guide is a MUST READ: http://www.vemssupport.com/forum/index.php/topic,97.0.html
Do you have a scope trace of this sensor output ?Falling edge is the more accurate edge when it comes to hall effect sensors as rise time is variable with hall effect sensors so trigger point will move (only slightly though)