Hi Open Des,
Since the default EO Acceleration Altitude is 1500ft AAL, I've assumed the macg calculated is the average value up to that AA.
A higher AA would need to be considered carefully, and if there's any doubt, then the best option would be to fly the EO procedure for the take off case and advise ATC accordingly. Fortunately, most of us will not see this problem during our entire aviation career.
I can remember a Trident in the 70s doing an EO GA from Madrid in summer and it didn't make 2.1% climb gradient. Fortunately they were VMC and managed to avoid all obstacles. See post 52 by Blind Pew
here
The problem was grasped by our performance people who analysed all our Destination and Alternate airfields and produced tables of planned maximum Landing weights for each runway with WAT limits.