[LAU] synchronization of two RME MADI cards

Dear linux-multichannel-audio-users, we are running a setup with two RME MADI cards to drive a system with approx. 100 playback channels. We are using jack2 and zita-j2a to achieve a sufficient number of playback channels. The sample clocks are synchronized via word clock, therefor the resampling is deactivated in zita-j2a. On a similar setup (with two different card types) I noticed that the latency between the cards is fixed as long as jack/zita-j2a is running, however, the latency differs between starts of jack. This seems also be the case in the setup with two MADI cards. Is there a way on the software side to solve this problem? Or is there a way to measure the latency without access to the analog I/O (knowing the latency every time would be sufficient)? How are other people solving synchronized playback of >64 channels? Best, Giso