I just noticed the note at the bottom of the 'Skillion/Shed Roof Tool' dialog seems incorrect.
The note says:
Note: The soffit/eave overhang is measured from pitching to fascia on the rake (angle).
however it seems that the actual eave/overhang is measured horizontally from my observations of the results of the tool.
unless i am too tired and am confused.
Cheers
John