[amsat-bb] Proximity to grid line boundary

For my information, as I am not much of a rover, but as I may be doing a little local grid rove sometime next weekend by request, how far from a grid boundary can you be for it to count? I found a 4 corners grid boundary in my area, but it is way rural in a deep clump of vegetation with a dirt road a couple hundred feet away. What is the maximum deviation from an X .000000; Y.000000 location for it to be legal as a 4 grid location? I will probably do a much easier 2 grid line location, but just wondering.

Thanks much.

Tom Schuessler, N5HYP EM12ms