  In Papers I and II from the Megamaser Cosmology Project (MCP), we reported
initial observations of water masers in an accretion disk of a supermassive
black hole at the center of the galaxy UGC 3789, which gave an angular-diameter
distance to the galaxy and an estimate of Ho with 16% uncertainty. We have
since conducted more VLBI observations of the spatial-velocity structure of
these water masers, as well as continued monitoring of its spectrum to better
measure maser accelerations. These more extensive observations, combined with
improved modeling of the masers in the accretion disk of the central
supermassive black hole, confirm our previous results, but with signifcantly
improved accuracy. We find Ho = 68.9 +/- 7.1 km/s/Mpc; this estimate of Ho is
independent of other methods and is accurate to +/-10%, including sources of
systematic error. This places UGC 3789 at a distance of 49.6 +/- 5.1 Mpc, with
a central supermassive black hole of (1.16 +/- 0.12) x 10^7 Msun.
