You're on the right path.
If you take two wheels both 9" wide. The first is your stock wheels +18mm offset(use that as your baseline). and the second is the -12mm offset.
The new wheels will stick out 30mm more than the stock wheels, or roughly 1.25". The increased outer position will result in increased space between the wheel and the suspension. For the 10" wide wheels you will need to factor in 1/2 added wheel width on each side (inner clearance and outer postion)
So if you go to a 10" wide wheel with a -12 offset it will stick out about 42mm more..
+18mm offset (stock wheel) then the change in offset of 30mm put it at -12mm offset. Since you are now running a 10" wide wheel the outer edge will stick out another 1/2 inch Or about 12mm. While losing 1/2 on the inside.
If you take two wheels both 9" wide. The first is your stock wheels +18mm offset(use that as your baseline). and the second is the -12mm offset.
The new wheels will stick out 30mm more than the stock wheels, or roughly 1.25". The increased outer position will result in increased space between the wheel and the suspension. For the 10" wide wheels you will need to factor in 1/2 added wheel width on each side (inner clearance and outer postion)
So if you go to a 10" wide wheel with a -12 offset it will stick out about 42mm more..
+18mm offset (stock wheel) then the change in offset of 30mm put it at -12mm offset. Since you are now running a 10" wide wheel the outer edge will stick out another 1/2 inch Or about 12mm. While losing 1/2 on the inside.