here is a link to the PDF that has the Echo info. Page 23-24 of PDF has the weight and balance data and graph. Unable to post any graphics as no scanner.
http://www.aft.com.au/exams/bobtait/perf_supp.pdf
I plotted a weight of 2655kg and moment index of 666. Then plotted another point by adding 100kg of aux fuel. this gave a point of 2755kg and an index moment of 694 (100kg x 2800mm arm divided by 10000 giving 28 then added to the 666). I seem to get a weight of 2905kg before if passes out of the envelope, so a total weight of 250kg of fuel. the max fuel for aux tanks is 218kg (80gl) so I put the answer down as 215kg which was I thought the closest
The Bob Tait book states adding weight in the forward limit range can only be done with the chart and not mathematically due to the limit moving as the weight changes