Unprecedented charged particle momentum resolution is required for precision Higgs studies at the International Linear Collider (ILC), which in turn demands as many as 200 sampling points with a high spatial resolution of 100 microns or better if we are to adopt a TPC for the central tracker. We discuss a novel theoretical resolution formula for a GEM-readout TPC, which is applicable to inclined tracks as opposed to the previous formula which is valid only for normal incidence. The formula identifies key factors that determine the spatial resolution and helps optimize the readout pad geometry and High Voltage settings for a given gas mixture. The formula is compared to the latest beam test results for a LC TPC Large prototype.