Hi Andreas,
Antenna delay varies on where or how the module or IC is used. The default antenna delay used with DecaRanging (515.371ns) is a good starting point.
It will also vary slightly from PCB design to PCB design or module to module, the equivalent of a few centimeters, so depending on the accuracy one requires you could program the same value into each design, module (slightly less accurate) or program them individually (more accurate).
Attached our Calibration application note APS014. Have a look at chapter 2.3.2 on page 11(14) , this will explain the above.
/Leo