There's no last number in 0.99999999999... but there's a last number in 1.0. That just proves that it's incorrect.
And no, the formula is used incorrectly. When you use it to find the fraction of a recurring decimal, you have to isolate the non-recurring part from the recurring part, by multiplying by an integer multiple of 10. In 0.999999... the recurring and non-recurring can not be separated, and so the formula is not usable.

