Given a dataset of N values, how to calculate the position(index) of the value at x percentile

Question

Problem is in the title.

The formula usually given for this is: Required Index: $\frac{x}{100}\times (N+1)$

I don't understand how this is derived.

why should it not instead be $\frac{Nx}{100}+1$ as $\frac{Nx}{100}$ values are behind the required value ?