another offset and backspacing question

ok i'm about to buy some 20's for my bagged 99 stepside s10, but i don't know what offset and backspacing i need. here is the site for the wheels that has the size chart http://www.americanracing.com/wheels/details.asp?parent=178&wheelid=177&name=RT-S%20Black&style=311&desc=1-piece%20painted/machined%20alloy&section=N

Someone please help me and tell me what i need for my truck.