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I find 39 up front and 36 in the rear works for me with DW tires based on the tire temperatures. What it says on the door is a good first approximation.
I have an '06 JCW cabriolet. Factory specs for the 215/40-18's are 36/36. I run Ventus V12's (awesome tire, by the way) in the 215/40-18 size and run 38 front, 36 rear. 'Works beautifully.