Hi David.

Welcome to the RME user forum.

I think you can neglect that. Have a look at the Octamic XTC manual.
Chapter 10: 3 samples for digital routing of audio files from MADI device to MADI device.
Chapter 27.3: this is lower than the converter latency, which is 13 samples for the XTC at 44.1 kHz.

Whether ADAT adds one sample more or less for the digital transmission is negligible.

At the end of the day, no ADAT driver in the world can tell you exactly, or determine for itself, which devices are connected to which converters. Or how many MADI devices are connected to a MADI bus, to be able to calculate the exact delay for the latency compensation.

At least, when connecting several MADI devices in series, you can use the RME MADI feature “delay compensation”.
This to ensure that audio from all devices connected to the MADI bus have the same delay, so that, for example, in stereo miking, you can connect the two mics for the right and left channel to any port on any device in the chain and connect several MADI devices to one bus without the left and right channel being delayed by a few samples from each other.
Delay Compensation is an RME specific MADI feature.

In the DAW, you have to make sure that you manually enter a latency compensation for these MADI tracks so that all tracks have the same time base again. A few samples difference might not necessarily be critical. After all, musicians on stage do not consist of a point source with the same distance to you.

For ADAT tracks, you would also need to calculate the compensation. Maybe for each of the ADAT ports, if you connected several (2) devices, if each of the devices has other converters.

An ASIO driver can only deliver the exact latency compensation for analog ports because converter latency for A/D and D/A is known to the vendor.