The "offset" is the difference between the centre line of the wheel and the
back of the hub, or the mating surface as you call it.
The distance from the back of the hub to the straight edge across the inner
rim is NOT the offset, but I have heard it called the "rear depth" or the "T
dimension".

I have here a mid 1980s ARG factory Mayfair alloy, part number NAM6071.
It is marked 4.50B x 12 x 33, suggesting that the offset might be 33mm.

I just measured the T dimension as 104 mm.

This is a 4.5 inch wheel, but that is the inside width of the channel that
the tyre sits in. The wheel has a tyre on it, which makes it hard to measure
the outside width accurately, but as a rule the outside width of an alloy
wheel is about 1 inch more, so we just add an inch.

If the outside width is 5.5 inches, and 1 inch = 25.45mm, the width is 140mm
, and the centre line of the wheel will be 70mm from the back edge.

The T dimension or rear depth was measured as 104mm.

Offset is centre line minus depth, i.e. 70 - 104 = -34,
so we have calculated the offset as -34 mm.

Which is pretty close to 33, and suggests that my guesstimate at the total
width - just add an inch - is out by one ot two mm.

So we conclude these wheels have a negative offset of 33mm.

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