[bobetter]

Actually, there exists a NMEA standard for this case and deals with WP arrival ( or WP passed as in your case) by using the perpendicular of the course line

AAM - Waypoint Arrival Alarm

Status of arrival (entering the arrival circle, or passing the
perpendicular of the course line) at waypoint c--c.

$--AAM,A,A,x.x,N,c--c*hh<CR><LF>
| | | | |
| | | | +---------------Waypoint ID
| | | +------------------Units of radius, nautical miles
| | +---------------------Arrival circle radius
| +------------------------Status: A = perpendicular passed at waypoint
+--------------------------Status: A = arrival circled entered