Correct Offset/Backspacing for 75 FS/SB?

[TIMSPEED]

Which offset/backspacing would I need if I wanted to go with an 18x9½" rim? I know that my tires will be 275/45 18's (same tire/rim size as Ford Lightning's) The rims I want are the Eagle Alloy 212's. For offset/backspacing, my choices are:
-18mm/4½"
-6mm/5"
+6mm/5½"
Which one would I need to go with? Thanks a LOT guys!