You need to calculate it yourself, or use the distance scale on your lens. (if it has one)
That flash (or any flash) does not show you the distance to the subject.
When used in the hot shoe, the flash takes ISO and aperture information from the camera to calculate the acceptable distance the flash can reach.
If set to TTL Mode, it will show you the “flash control range” which means the subject should not be closer, or farther, than what is displayed.
In Manual Mode, it will show you the “optimal shooting distance” which means the subject should be at the distance displayed in order to have correct exposure.
When removed from the hot shoe, or in bounce mode, the distance values disappear because there is no way to calculate the distance needed for proper exposure.