  We present a method that optimizes the aspect ratio of a spatially
anisotropic quantum lattice model during the quantum Monte Carlo simulation,
and realizes the virtually isotropic lattice automatically. The anisotropy is
removed by using the Robbins-Monro algorithm based on the correlation length in
each direction. The method allows for comparing directly the value of critical
amplitude among different anisotropic models, and identifying the universality
more precisely. We apply our method to the staggered dimer antiferromagnetic
Heisenberg model and demonstrate that the apparent non-universal behavior is
attributed mainly to the strong size correction of the effective aspect ratio
due to the existence of the cubic interaction.
