Our tandem team weighs more than twice the OP, and the Silca calculator only recommends 87 psi for 28mm on the tandem.
Basically, for optimal comfort and reasonable rolling resistance, you want to run pretty much the lowest pressure you can. The limits on going too low, are 1), as noted pinch flats, and 2) the tire squirming. If you’re not getting pinch flats, and the tire doesn’t squirm as you corner, then in all likelihood your pressure isn’t too low.
You can get more scientific, calculate the pressure that gives you the “ideal” amount of drop, research rolling resistance for your tire with various combinations of rider weight and pressure. But if the bike rides well, doesn’t squirm and doesn’t pinch flat, you really don’t need to sweat it further.
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You could get lost and die.
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