The speed of light (radio waves) in a vacuum is 299792458 meters per second. So in 1 millisecond a radiowave travels 299792,458 meters, or just short of 300km. In order to correct for this delay, you have to configure your distance to the DCF77 transmitter. My receiver is 385km away from the transmitter site. And the delay is: 385 / 299,792 = 1,284 millisecond. I don't know if Meinberg corrects for the difference between air and the perfect vacuum
Site of the DCF77 transmitter is the transmitting radio station Mainflingen (coordinates: 50°01' north, 09°00' east) approx. 25 km south-eastern of Frankfurt/Main.
You can find sites on the internet that calculate the great-circle distance.