You've definitely got the idea. The calculations are cumbersome, but not too difficult.Nulls are formed as pockets of areas where certain frequencies are attenuated by out of phase cancellation. The edges of these nulls are fuzzy, as the phase angle nears 180o and cancellation begins to increase towards the center of the null area. The center is where cancellation is the greatest. At the center, a very sharp notch is formed. Surrounding this is a broader area of general amplitude depression.