From a comparison of the Mariner 10 third-encounter UV spectrometer data with intensities generated from a newly constructed model exosphere, we have derived a new value of 4.5¿10-4 for the fraction of the solar wind He++ flux to be intercepted and captured by Mercury's magnetosphere if the observed He atmosphere is maintained by the solar wind. If an internal source for He prevails, the corresponding upper bound for the global outgassing rate is estimated to be 4.5¿1022 s-1. These values differ from those given earlier because of the present use of a surface temperature distribution satisfying the heat equation over Mercury's entire surface which employs Mariner 10 determined mean surface thermal characteristics. We also use the mean standoff distance of Mercury's magnetopause averaged over Mercury's orbit. We find the agreement between the observed and calculated intensities to be good and believe that the minor discrepancies that exist on the nightside of the terminator are explicable in terms of difference between actual and computed surface temperatures and the resulting scale height structure changes. We attribute these temperature differences to the inhomogeneity of the physical properties of the surface of Mercury.