To give yourself a ballpark figure, try this.
Clamp a straight edge on the wheel mounting face of the hub and measure the dimension from that face to the strut tube, just below the lower spring seat.
The maximum backspace is 10mm
less than the above measurement.
Dimension "A" in the formula
That gives a wee bit of space for the bulge, if any, of the tyre sidewall.
Now measure outwards of the wheel mounting face to a point approx. 15-20mm inside of the guard lip viewed from above.
That will give you the overall width of the rim.
Dimension "X" in the formula.
Specified [manufactured] rim width is approx, 20-25mm
less to account for the thickness of the alloy rim.
e.g. if you measure 170mm, the rim width is 150mm or 6".
Then you can use the magic formula in the Conversion Chart above to calculate the approx. wheel offset, "B" in the formula