I’m 75 kg body weight and run my Monty Pro Race tyre at 38 PSI on my trials bike, that gives me bounce and prevents bashing the rim when landing on sharp edges. My reference here is “the lowest pressure that prevents rim damage when landing on sharp edges”. In trials tyres are usually under-inflated compared with road bikes.
For non-trials bikes, my rule of thumb is to check the pressure rating written on the side of the tyre, and inflate the tyre to that value minus a couple PSI, then gradually release air incrementally, testing in between releases, until the vibrations from the road no longer feel uncomfortable. In this case the reference is “the highest pressure that that feels comfortable”. You don’t want to release too much air because an under-inflated tyres wear faster and require more energy ride. If you are riding indoors on super-smooth floors you probably can get away with higher; on the road you’ll probably want a lower value.