Converting 1000 km to 600 miles is always wrong. Assuming that 1000km is not an exact figure solely because its least significant digits are zero is also wrong,

Converting 1000 km to 600 miles is always wrong. Assuming that 1000km is not an exact figure solely because its least significant digits are zero is also wrong, and making up a ±10% margin is very wrong. The correct thing to do would be to write the conversion as "1000 km (~621 miles)", so that the original value is not lost.