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I have 20's also. And your telling me that you should run your tires on what the door says. Even though the side wall height and size are completely different than what came on it stock?
In this particular case, the OP really should start out with 32 psi. The 255/40-20 carries the same load index rating as the OE 235/50-18's (97). It's when the load index for your new tires differs from the load index for your OE tires that you start thinking about starting at different pressures.
BTW, once you start to deviate from the sticker pressure, there is no reason to keep all four tires at the same pressure. Or at the same pressure difference if your fronts and rears are spec'ed for different pressures.
