That's really all you need for SOS, as you get an area almost 1 degree from the actual map coordinates to fish it up.
For T-maps, that will get you in the area, as 6 steps (100 mining) is about 0' 24", so if the coordinates are truncated instead of rounded, you'll be at worst 10 steps diagonally from the map. If they are rounded, then that becomes a 15x15 grid centered on the lat/long
Without mining, truncated numbers mean that you end up with a 15 x 15 area to search with the start point at one corner on the square (which corner depends on if it's N/E, S/E, S/W, or N/W), which isn't unreasonable (given that the game effectively has magical GPS via enchanted sextants and even a command to give your coordinates, if you know it). Rounded numbers just center the square on the coordinates, and might be closer to being a 17x17 square.
Anyone remember whether the locker rounds or truncates coordinates?