Computing by code is the minimal standard. Go for the 400 Amp. It is my experience that owners of these custom homes count the size of the service as a matter of pride. "My house has a bigger service than your house".
You want the job? Tell the owner according to your calculations, he will need to upgrade to 400 Amps before the decade is out, so you suggest he has you install 600 Amps, just to be safe. (with an automatic back-up emergency generator, too)
Earl
