These cameras use scale focusing, so accurate focus depends on estimating or measuring subject distance and setting that distance on the lens.

For portraits, the most reliable improvement is to use a small laser rangefinder. The inexpensive short-range models sold for home improvement are suitable and can give you a much better distance reading than a phone app.

To make focusing less critical, use more depth of field: stop down the lens when light allows. If that leads to shutter speeds that are too slow for handheld shooting, use faster film so you can keep a narrower aperture without introducing camera shake.

In short: measure distance with a laser rangefinder, and combine that with higher-ISO film and a narrower aperture to increase your margin for focus error. That’s the standard practical approach with scale-focus cameras.