Drop a string and plumb bob from the point you want to be at, and measure to the hub face. That gives you a near offset measurement, you will also need rim width to account as well for your total ET backspacing. Just remember the wider the lower. I went from a 18x7 et43 to a 18x8 et32 and I'm about flush. Technically the extra inch all went outward with this so my strut clearance is the same. There is a bunch of different wheel size comparison calcs on line if you find a wheel you like to compare your current specs to new. Nothing worse than finding new wheels won't fit and additional spacers needed.