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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.)

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    There may not be a single answer. You need to consider riding conditions, tire size and material. A bare minimum pressure is when the tire remains round under rider's weight. A pressure under minimum isn't "wrong", the tire just wasn't made for it. For example sidewalls will eventually break when riding with too low pressure. – mkpaa Dec 19 '18 at 01:59
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    Some setups will run happily at 10 PSI, and others will still bottom out at 90 PSI. The only rule of thumb is "if you're getting pinch flats your tyres are too low, add air" – Criggie Dec 19 '18 at 02:28
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    Pressure printed on tires is merely a suggestion for values from which to start tweaking it your liking. You can go as low as you deem safe. Sure, some tires like Conti GP4000 don't wear well at very low pressure, but you can generally research forums on that. – Klaster_1 Нет войне Dec 19 '18 at 03:25
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    80% is way too much. My gravel tyres are rated 45-70. That is with inner tubes. I run them 35-40 tubeless and less when I forget to pump up. In a known terrain without stones, roots and other edges 25 would still work. – Vladimir F Героям слава Apr 10 '20 at 18:31

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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:

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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.

Batman
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    It's interesting that that table doesn't suggest running a bit higher pressure in the back tyre - when I ran 35s front and rear I had about 5psi before their value at the front, and 5 above at the back (still within the rating, and that was the optimum I only worried about for long rides). I'm still figuring out what I prefer with a 32 on the front, but these pressures don't completely protect from pinch flats as I found a week ago – Chris H Dec 19 '18 at 08:18
  • With the pressure in bar and rider's weight in kilogram, the rule of the thumb for a 25mm tyre suggests: Weight - 10% then divided by 10 for pressure. Thus a 75kg rider may ride at 6.5 bar.. – Carel Dec 19 '18 at 08:37
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    Acording to the guide I would be somewhere around 55-60 PSI for 40 mm(38C). That is too much for off-road. Almost unrideable. The Schwalbe tyres I have are rated from 45 PSI, which is much better but it rides much better tubeless at 35-40. Once inadvertently (or stupidly) below 20 I pinched the tyre. – Vladimir F Героям слава Apr 10 '20 at 18:37