From what I've read the acceptable offset is between 35mm and 42mm.
Here's a link which explains backspace and offset:
http://www.rx7turboturbo.com/robrobi...eel_offset.htm
and here's a link where you can calculate backspacing/offset from a reference wheel to a new wheel:
http://cobras.worldwidegarage.com/in...20Stuff&meid=3