Offset is the best dimension to use. The best way to figure that out is to have the tire off of the wheel for measurement. You simply measure the overall wheel width and divide by 2 for the center line. You can then use the backspace to determine the offset of that wheel.
If the tire is still on, then you can make some assumptions to ballpark the figure. You will need to know the wheel specification and that usually is stamped on the wheel. A wheel's overall width will be about 1" wider than the specification so a 5" width specification will have about a 6" overall width because the specification is measured between the beads. That and the measured backspace will get you in the ballpark for the offset of the wheel.
If you will be using tires of the same width then you don't have to do more but if you want to fit the widest:
The other thing you need to know is where the tire is located relative to the inner and outer fender, steering linkage, shocks , springs, etc. This can be determined simply by measuring. You can use that information to determine which way you need to shift the offset to get the wheel centered. Having the wheel centered will allow you to fit the widest wheel and tire combo possible within the available space.
To simplify things for you, the 7"W, 4"BS wheel will move the tire 1/2" closer to the inner fender and 1 1/2" closer to the outer fender vs a 5"W 3.5"BS wheel, everything else equal. Measure and see if that will work.