Magnetic field strengths in objects ranging from HII regions to cosmological large scale structure can be estimated using dense grids of Rotation Measures (RMs) from polarized background radio structures. Upcoming surveys on the SKA and its precursors will dramatically increase the number N of background sources. However, detectable magnetic field strengths will scale only as t−0.15, for an integration time t on a fixed area of sky, so the analysis techniques need to be optimized. A key factor is the difference in the dispersion of intrinsic RMs for different populations, which must be carefully accounted for to achieve the scientifically needed accuracies.