Calculating Waypoints.

I've been using the GPRMC data, from my gps unit.

When I want to record a waypoint, I save the Lat/Lon data from the GPRMC string.
But you do not always get an exact match, to the Lat/Lon the next time you
pass the same location. (GPS is not that accurate)

What is the best way to calculate a bigger waypoint window.
Is there a standard way to do this?

I have some code working but it's not 100% accurate.

Any help!