I suspect a little that it was loose at one point and there won't be much movement of the brake within the thru-axle's window of "good" torque values. In other words, the range given for allen-headed "stealth" type thru axles, the only type that readily take a torque wrench, is usually about 9-13Nm, and I doubt there would be movement of the brake without about that range.
Torque it so that it's firmly bottomed out and you can feel there's heavy resistance to further preloading of the threads. Make sure there's some grease on the threads to make it easier to sense what's going on. The nature of big coarse aluminum threads actually makes it pretty clear to tell through wrench feel when you've reached the "bottomed out" point.
If it's still vague and you're having trouble with the brake wanting to move, that's unfortunate. Make sure the hub isn't missing any hardware and that any removable bits of the dropouts are tight. Finally, you can use a marker to make an index mark to set the rotational position of the axle to every time. For example, on the drive side where the end of the axle is visible when installed, you could draw a line across its face and onto the dropouts. You still have to make sure it's tight when you reinstall it in case something shifts/wears, but for the most part that should make it easy to reproduce the "adjustment."