What psi should i put in my tires?

I have a honda prelude which originally comes with 205/50R16 tires. I upgraded to 225/40R18, and i paid freaking $600 so they can be professionally done at firestone. When i rolled out i realized that the front tires are kind of bulging and looking deflated(maybe due to the weight of the engine) but the back are looking fine. I went back and asked if the psi they put is fine since i'm worried the low looking psi would destroy my tires or even worse my rims when i hit a bump. The guy told me i'm supposed to put the psi on the placard(at the penger door). I'm pretty sure that's for the 205/50R16 tires not my after market 225/40R18. My after market tires say max psi should be 50 on the side. Would it be ok if i put it to around 40 psi to stop the bulging and looking deflated or was the guy correct?