Front wheel backspacing with 11.75 inch rotors ??

Hi,

I'm finishing the installation of a set of 11.75 inch rotors using the Magnum force spindles and the rest '73 disc components on the front of my '68 Dart. I'd like to go with 16 x 7 wheels and 225/50-16 tires. What backspacing on these wheels do I need to get them to fit under the fenders..?? Would 4 inch backspacing be OK or do I need more than that ?

Thanks,

Mark.