  We measure F814W Surface Brightness Fluctuations (SBF) for a sample of
distant shell galaxies with radial velocities ranging from 4000 to 8000 km/s.
The distance at galaxies is then evaluated by using the SBF method. For this
purpose, theoretical SBF magnitudes for the ACS@HST filters are computed for
single burst stellar populations covering a wide range of ages (t=1.5-14 Gyr)
and metallicities (Z=0.008-0.04). Using these stellar population models we
provide the first $\bar{M}_{F814W}$ versus $(F475W-F814W)_0$ calibration and we
extend the previous I-band versus $(B-I)_0$ color relation to colors
$(B-I)_{0}\leq 2.0$ mag. Coupling our SBF measurements with the theoretical
calibration we derive distances with a statistical uncertainty of $\sim 8%$,
and systematic error of $\sim 6 %$. The procedure developed to analyze data
ensures that the indetermination due to possible unmasked residual shells is
well below $\sim 12 %$. The results suggest that \emph{optical} SBFs can be
measured at $d \geq 100 Mpc$ with ACS@HST imaging. SBF-based distances coupled
with recession velocities corrected for peculiar motion, allow us obtain $H_{0}
= 76 \pm 6$ (statistical) $\pm 5$ (systematic) km/s/Mpc.
