From this code, it seems that the fixed latency value should be always configured in units of 8ns https://ohwr.org/project/wr-cores/blob/proposed_master/modules/wr_streamers/fixed_latency_ts_match.vhd#L152

while it seems that it might depend on the system clock (16ns for 62.5MHz, 8ns for 125MHz). This is what we observed with John in the wr2rf board. TO BE INVESTIGATED