jiangshi, thank you very much! 2 beers for you, if we ever meet! I think the Italian and English should show the same offset - the only difference is that the part that's inside the BB shell is 2 mm longer for the Italian than for the English. And that 2 mm should be evenly distributed.

I would measure the difference in stub lengths from the BB shell edge to the end of the square taper, of course with the crank arms removed. This is actually how SER (shell to end, right) is defined in Sutherland's 4, 5, and 6th editions. Then I calculate SEL by subtracting SER and BB shell width from the spindle length.