i don't like the poke of 255/40/18 at ET30 up front. 255/40/18 will not rub at ET40 however it may be more tucked than you desire
i'd get the ET40 and determine the spacer size after it is installed and lowered. the spacer size will probably be between 3mm-7.5mm and will be different between the front and rear. you can determine thickness by plumbing a string or tape and measuring the distance from the string/tape to tires upper sidewall/shoulder. that distance will be the spacer thickness if you desire the sidewall flush. spacer thickness less than measurement=slightly tucked, more=slighlty poked, this should be obvious but jus sayin'...
in the link below johnung has 255/40/18 @ ET40 with 3mm front, 5mm rear on his lowered F30. if you like the look go with the same. if you want more aggressive 5mm front, 7.5mm rear. personally i'd wait till you get yours and measure your setup specifically as this would be most accurate.
https://f30.bimmerpost.com/forums/sh...00&postcount=2