I was looking for a good video to post and the TSW one does a reasonably good job. In the TSW example, if you go from ET35 to ET40, you are removing 5mm from the backpad on the wheel and the wheel is going to move 5 mm inboard.
The offset and width are both important measurements as the offset is based on the centerline of the wheel but the width determines how far out the wheel bead extends from that centerline.
You can easily build a calculator in Excel but this online tool does a good job based on tire size, wheel width and offset to see how different wheels fit:
https://www.customwheeloffset.com/ti...et-calculators