working out offset or ET

hi i have a set of ats cups and need to work out the offset to replace one that i bent

a minus offset push's the wheel wider out of the arch i assume
if i knew the width of my wheel the middle would be et0 ??

so then i need to work out the distance from the hub flange to the inside of wheel edge and then what ??
any help would be good
cheers steve

 

https://www.rsracing.com/tech-wheel.html

 

cheers good site

 

Here's a good wheel offset calculator to demonstrate how wheel width and wheel offset affect wheel positioning.