If you have the exterior microphone intended for rimfire or suppressed firearms, just use it. Set it under the barrel a few inches back from the muzzle. This is what I do now for every rifle and pistol for which I want to gather data. Way easier than moving the unit around.
You need to ascertain whether your brake directs gas to the side or to more to the rear. If the baffles are angled about 90 degrees to the barrel axis, it's probably safe to put the unit 6" back. If the baffles are angled further back, I would put it just in front of the scope turrets. If I put mine 6" back of my APA Little B*trd brake, the unit would catch some blast. Bad readings, eventual damage.
If you can shoot under the unit via a stool or tripod, that's good too - you'd really want to use the phone app in this case.