inorganic chemistry - Outermost electronic configuration of f block elements

Why is it $(n-2)\mathrm{f}^{1-14}(n-1)\mathrm{d}^{0-1}n\mathrm{s}^2$.

The only exception I thought to this formula was Thorium $\ce{5f^{0} 6d^{2} 7s^{2}}$.

So I feel it should be $(n-2)\mathrm{f}^{1-14}(n-1)\mathrm{d}^{0-2}n\mathrm{s}^2$.

Answer

As you said, if you look at the electron configurations of Lanthanides and Actinides, Thorium seems to be the only exception from this empirical rule. You could expand the rule to also include the case of Thorium but I would say it is more useful to have the rule describe the majority of cases and remember $\ce{Th}$ as an exception.