Sometimes there is a lot of guesswork involved on making some adjustments. Steering cables need to be tight, but not too tight. Fittings need to be secure, yet it is impractical to put a torque wrench on every nut and bolt. One mystical adjustment is engine v-belt tension. Too loose, the belt wears, sprays dust everywhere, over heats accessories, and doesn’t drive the water pump or alternator efficiently. Too tight and the v-belt puts excessive tension on the bearings and pulleys, wears quickly, and over heats accessories.
How do you find that goldilocks tension? The standard advice is to press on the belt until it feels right and depresses the belt about 3/8”. That at best is a crude measure as it depends on the strength of the hand depressing the belt. Not to fear Gates, the v-belt manufacturer makes an inexpensive gauge, the Krikit I V-belt Tension gauge. Hook it on the belt, press until it clicks, and then read the tension in pounds and kilograms.
So how much tension is enough tension? The instructions suggest reading the engine’s manual and following their recommendations. For the JH(2) series Yanmar recommends a 10mm depression with 10 kg of pressure on the section between the water pump and alternator. Other manufacturers may have different settings.