In the nearby galaxy NGC4258, the well-modeled orbital motion of H_2_O masers about its supermassive black hole provides the means to measure a precise geometric distance. As a result, NGC4258 is one of a few "geometric anchors" available to calibrate the true luminosities of stellar distance indicators such as the Tip of the Red Giant Branch (TRGB) or the Cepheid Leavitt law. In this paper, we present a detailed study of the apparent magnitude of the TRGB within NGC4258, using publicly available HST observations optimally situated in the unreddened stellar halo along the minor axis, spanning distances ranging from 8 to 22kpc in projected galactocentric radius, or 6' (13kpc) to 30' (66kpc) in distance along the semimajor axis. We undertake a systematic evaluation of the uncertainties associated with measuring the TRGB in this galaxy, based on an analysis of 54arcmin^2^ of HST/ACS imaging. After quantifying these uncertainties, we measure the TRGB in NGC4258 to be F814W0=25.347{+/-}0.014(stat){+/-}0.042(sys)mag. Combined with a recent 1.5% megamaser distance to NGC4258, we determine the absolute luminosity of the TRGB to be M_F814W_^TRGB^-4.050{+/-}0.028(stat){+/-}0.048(sys)mag. This new calibration agrees to better than 1% with an independent calibration presented in Freedman et al. that was based on detached eclipsing binaries (DEBs) located in the LMC.