100 keV H + medium energy ion scattering (MEIS) blocking patterns have been measured using incidence in the <210> and <310> directions from the Ni (100)c(2 x 2)-O surface phase. These data formed the basis of a structure determination through comparison with simulated data for different model structures. A detailed evaluation has been made of different criteria for establishing the best-fit theoretical simulations, using R-factors based on chi-squared, from previous MEIS studies, and from other surface crystalloraphic techniques (LEED and photoelectron diffraction). A similar evaluation of criteria for establishing significance and precision is presented.