Too much positive offset will make you hit your coilovers or chassis, the more negative offset you have will make your wheel stick out.
Take note that the width of the rim plays a major roll on your offset. The bigger the rim the less room you have to play with offset sizes.
Conversion: 1 INCH = 25.4mm
example: 17x10 rim is 254mm wide (0 offset for that gives you 127mm sticking out and 127mm going in)
Mods sticky this?
Also please feel free to throw in some exact figures in here as far as whats the maximum clearance you have gotten from rim backspace. Thanks!