Measure the backspace of your current front rims by putting a straightedge across the back of the rim, and measuring to the hub flange. Install the rims on the car and turn the steering to left lock, measure the distance between left tire and frame. Turn to right lock, and do the same. measure both sides, both positions. Take the smallest distance, and add to your current backspace measurement. Subtract 1/2" for clearence/sidewall flex, and that should be the maximum backspace you can fit.
For fender clearence, you need to know how wide your centerline is, and subtract its backspace. That is how far it sticks out past the hub. Determine how much further, if any, another rim could stick out without touching the fender at any time. This will allow you to determine how wide you can go, and your offset, with respect to backspacing.
With all of these measurements, you should be able to determine maximum backspace and width of rim and tire that you can fit. Always go a little thinner to compensate for sidewall flex.
The backspace measurement can be the same for any width rim and clear the frame, but you have to consider how far you want the rim to stick in or out (offset), so you use the maximum backspace measurement as a guideline/safety.
