Dipole moment - calculation of percentage ionic character
The calculated dipole moment is charge on electron * radius of
  molecule


That's off a bit, the calculated dipole moment (calculated assuming one electron has been completely transferred from hydrogen to bromine) is dependent on the bond length (interatomic spacing), not the radius.
\begin{aligned}
\mu_{100\%} &amp;= \mathrm{electron ~charge \times interatomic ~spacing}\\
\mu_{100\%} &amp;= (1.6 \times 10^{-19})\ \mathrm C \times (1.41 \times 10^{-10})\ \mathrm m\\
\mu_{100\%} &amp;= 2.26 \times 10^{-29}\ \mathrm{C\ m}\\
\end{aligned}

As you point out,
$$\mathrm{\% ~ionic~ character} = \frac{[\mu_\mathrm{obs}]}{[\mu_{100\ \%}]} = \frac{[2.6 \times 10^{-30}]}{[2.26 \times 10^{-29}]} = 0.12 = 12\ \%$$