Standard Cougar offset/ET on the OEM 16" diameter, 6.5" wide alloys is 46. As you increase the width of the wheel to 7", 7.5", 8"* etc the ET should DECREASE to 40, 38, 36* etc to ensure the wheel and tyre stay inside the arch and to maintain the tracking width as close as poss. to standard.
*The figures quoted don't have to correlate exactly but you get the idea.