the first number is tire width, the second one is thickness of the tires and the last is the diameter. if you were running 235/40 before i actually think you could run 225/40 in the rear and not rub. i don't know why you are running a staggered setup on a front wheel drive car, but im not here to judge. imo i actually think its rather stupid to run a staggered setup on a front wheel drive car because all of your contact patch needs to be in the front wheels not your rears. maybe you purchased the car like this?
215/35/18 is perfectly fine for the rear, but the roads in texas suck so i would be scared to run a 35 series tires without bending a rim.
my advice would be just to keep the tires you have now until you need 4 new ones and get some 224/40/18s all around.