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|
|130 lbs (59 kg)||185 lbs (83 kg)||240 lbs (109 kg)|
|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.