Would you know how to determine this point electrically, please?
The inductance of the primary and secondary abruptly change at the point where the magnetic flux reverses in the armature. As you approach that point the flux lines get forced closer together so to torque required to turn the armature increases. At the point where it reverses the torque also abruptly decreases, making it difficult to stop turning the armature when that point is reached. It's like pushing on something that is stuck and trying to stop pushing at the moment it becomes unstuck so that the object remains in the same position without having moved.
Anyway, a protractor attached to the armature and an inductance meter attached to the coil lets you plot L vs. angle. Although you have the same problem of torque abruptly decreasing at the flux reversal point, fitting the curves before and after and extrapolating to where they cross lets you determine the point to better than 1-deg. without too much effort.