Reflexivity defect of kernels of the elementary operators of length 2

Let X be a finite-dimensional complex vector space. We give an explicit formula for the reflexivity defect of the kernel of an arbitrary elementary operator of length 2, i.e., an elementary operator of the form Δ(T)=A1TB1-A2TB2(T∈L(X)) where A1,A2 and B1,B2 are linearly independent.