Some tires only have the maximum pressure written on them, stating nothing about the minimum. For soft and comfortable rides, the recommendation is to inflate them to lower pressures, but then how to avoid under-inflation? Should we just not overthink it and stick to a reasonable value, such as 80% of the maximum pressure? (That's what I would estimate based on a few pressure charts I have seen.)
Smaller tires need to be run at higher pressures to accommodate the same weight. You want to have enough air that the tire's rolling resistance is good but you don't get pinch flats / damage from appropriate road hazards. Too little/much air can affect control to some extent as well. If you have tubeless tires, you can usually run lower pressures (since there is no tube to pinch flat); your rims may or may not be setup for this.
If the tire isn't deflecting, the tire is overinflated. You should see some (but not a huge) amount of deflection when the tire is loaded. Here are some pictures from Schwalbe:
I'd start with some guide like this one from Schwalbe as a guideline (interpolate/extrapolate for your body weight/tire size, and move up / down if it seems over/under inflated)
Tire width Body weight approx. 130 lbs approx. 185 lbs approx. 240 lbs 25 mm 85 psi 100 psi 115 psi 28 mm 80 psi 95 psi 110 psi 32 mm 65 psi 80 psi 95 psi 37 mm 55 psi 70 psi 80 psi 40 mm 50 psi 65 psi 80 psi 47 mm 45 psi 55 psi 70 psi 50 mm 35 psi 55 psi 70 psi 55 mm 30 psi 45 psi 55 psi 60 mm 30 psi 45 psi 55 psi
80% of the sidewall pressure may be too high/low for you; it depends on how heavy you are and what that manufacturer put on the sidewall. However, once you pump up your tires a few times and get a feeling for what tire pressures work for you for your tire sizes, you won't really worry and will have a natural feel for it.