The “focus one-third into the scene” rule is only a rough myth. Hyperfocal distance depends on focal length and aperture, so it changes from shot to shot.

Without a distance scale, practical options are:

• Use a hyperfocal chart or calculator for your focal length and f-stop, then estimate that distance in the scene.
• Practice estimating distances by eye; this gets easier with experience.
• Use live view, magnify the image, and manually focus while checking foreground and background sharpness.
• If your camera has a depth-of-field preview button, stop the lens down and evaluate the range of acceptable sharpness as you adjust focus.

A good general approach for landscapes is to stop down to around f/8 or f/11, then focus at the estimated hyperfocal distance rather than at infinity or arbitrarily one-third in.

Also remember that “maximum sharpness from front to back” has limits: if the foreground is very close, one frame may not keep everything perfectly sharp, especially with a wide aperture. In those cases, stopping down more or changing composition/distance may help.