minimum clearance to springs?

hey guys, i just ordered some 15X8 withs 4.5 inch backspacing with some 275/60-15s for my SBP 8 3/4 duster

they clear the springs...barely
im thinking of adding some spacers to ensure it wont rub on the springs during cornering BUT i dont want to add more then i need to, because i like as much threads on my lugnuts as possible

so, what would be the minimum clearance i want between my springs and tires?