Outer Limits 4x4 Board

Can anyone tell me the offset and how it is measured on 80 series 15*8 steel wheels.

I am looking at either a ROH with a neg 13 offset , a mangels with a neg 25 offset or a second hand set of 80 series wheels any thoughts?

Gday
The 80 didnt come with 15x8's, the 16x8's are 0 offset, or 4 inch backspace, so the oem 15x7's will also be 4 inch backspace or -13

I have +23 offset on my 94 80 series and they are perfect fit under the guards.....tyre sidewall is level with outer flare face. This offset is 60/75 series offset.
Andrew