Measurements in game
 

OK, the map is in kilometers.
Speed is in nautical miles per hour.
Uboat range in booklet expressed in nm (is that one way or return?)


1 nm = 1.85 Kilometer or 1 KM = about 1/2 nm

To convert map distance from KM to NM multiply by two, then divide by your speed to find time to that position.

On your waypoints it shows distance in KM and time in hours to get there at current speed.

If you are mixing surface and dived travel, you can get average speed by adding up the distance covered from start, convert to nm by x2, divide by hours travelled to get average speed.