If you have the default static crosshair and are fully zoomed in the numbers correspond to shell travel time for a ship moving perpendicular to your view at 20 knots.
If you have the dynamic crosshair enabled, the numbers on it correspond to the shell travel time for a ship moving perpendicular to your view at a speed of 30 knots. Make sure you have 'alternate battle interface' enabled, as this gives you a readout of the shell travel time for your aim point.
