I have a fixed focal length camera where it's pretty simple to do the Rule in one's head. Focal Length is 24.2mm, say 24mm. So at f/8 the aperture diameter is 24/8 = 3mm. Ergo, for someone with average vision, the Sh for 24mm and f/8 would be 3 x 1.5 = 4.5m = 15ft.
The Rule: Hyperfocal distance =
focal length divided by the f-number divided by the DOF fraction.
Remember, this a quick estimate without the need for tables or formulae or apps, so don't expect super accuracy but, for me, it's "good enough for Government Work" ...