I agree with your point about not trying to satisfy GB use case particularly as even the basic IATI dashboard hasn’t addressed coverage in a couple of years and cracking that nut is important. I would still point out that “total expenditure”, if not well defined or understood, could still be very misleading as a denominator in the coverage calculation. Total expenditure for an organization often includes expenditures never intended to be part of IATI publishing (administrative, normative) so without clarity or sub-elements, attributes (I don’t know the correct way to slice that) it may still lead to misleading results.
And just and FYI: the GB dashboard has not yet captured coverage at all. I have no idea if/when this will be addressed in the narrow context of the GB but we all score zero so it is a level playing field for the moment.