If you open the poles up too far, then the magnetic field between the poles may not be enough to detect the cracks you need to find.
If you keep them too close together, there are two risks
1. You will get a smaller coverage, so need to test next test much closer
2. You may saturate with magnetism and cause too big a background to see cracks.
Making your Yoke pole spacing too close will create too much sensitivity due to the increase in the magnetic field strenght and that creates unnecessary indications. On the otherhand, making your pole spacing too far would provide lesser magnetic field strenght (lesser sensitivity).
So it is better for you to established your own correct pole spacing by doing some experimentation using same material of the same thickness with a known artificial defect and with that you can make your own appropriate MT procedure.