  We compute individual distributions of low-lying eigenvalues of a chiral
random matrix ensemble interpolating symplectic and unitary symmetry classes by
the Nystr\"om-type method of evaluating the Fredholm Pfaffian and resolvents of
the quaternion kernel. The one-parameter family of these distributions are
shown to fit excellently the Dirac spectra of SU(2) lattice gauge theory with a
constant U(1) background or dynamically fluctuating U(1) gauge field, which
weakly breaks the pseudo-reality of the unperturbed SU(2) Dirac operator.
Observed linear dependence of the crossover parameter with the strength of U(1)
perturbations leads to precise determination of the pseudo-scalar decay
constant, as well as the chiral condensate in the effective chiral Lagrangian
of AI class.
