tire size question

I've read what seems like 10,000 posts on tires sizing in the past few days.....i think the more i read the more confused I am. 

im going to be running a 20x9 +29 and 20x10.5 +30 on my Ls430. I dont mind a mild stretch but dont want a crazy stretch. Im thinking 225/35 or maybe 235/35 up front and 255/35 in the back. Am I on the right track? I have a set of 245/35's that I was gonna try on the back, but just eyeing it up it looks like it will be stretched out more than I would want. My shop is telling me to go with a 40 side wall just to have more protection for daily driving, but that will obviously be counter productive for fitment as well.

Any input would be appreciated. Im by no means new to the car game, but my GS is all about power so Im running wide true running tires on it. 

thanks