while the iRig hd has no output one can connect a microphone to it instead of a guitar for the calibrate function to work.
Without a micophone one can click it but gets arbitrary results.
But the results seems or are wrong, because the values are too long. About 10ms off.
Manually setting the value to the displayed latency + selected latency seems to be the more accurate value.
It can not be set exactly though.
Strangely, the displayed device latency can obviously change while running.
Sure the irig hd is no longer sold it still works well.
Not clear what the issue is.