okay to get a fairly rough figure but not so good for the pin point accuracy they seem to ask for
I don't think that pinpoint accuracy is the target. Why don't you find someone with a scanner and post some of your exercises ? It may be that we can suss out something not quite suitable in your technique ?
the CG envelope is not suitable - the graph is distorted, and too small
Not so much distorted and small. The use of the OEM datum results in these dreadful envelopes. This is why most of the trimsheets you might see use a trim datum in, or near, to the envelope. The result is a much more useable, boxy envelope.
By my reckoning, about 169.5, call it 170 kg .. but this can't be obtained from the chart. If you prefer to be pedantic in conservatism, one might round down to 169 kg.