That CG envelope is not very suitable for plotting and the result will be a bit average no matter how careful you might try to be. However, if that's all you have, just use a sharp pencil and take a little care.
Then plotted another point by adding 100kg of aux fuel
That's just asking for trouble.
This involves extrapolation which magnifies plotting errors - especially when you are using poor quality charts and trying to get an intersection of two lines of similar slope. Always plot beyond what you need so that you are interpolating - that way the effects of plotting errors are minimised. I would have plotted a point outside the envelope so that the point to be read off is between the two plotted positions. For example you could have used, say, full aux fuel to get the plotted point. Much better strategy and one I would have thought Bob would have recommended to his students.
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
For the upper forward limit, that's a practical and pragmatic approach although the errors are significant for lines of similar slope. The calculation approach involves simultaneous equations which, while not being at all difficult, is not really relevant to the pilot's work.
One trap there, and not generally understood, is that the upper forward CG line is a straight line on the CG envelope but a quadratic curve, not a straight line, on the moment envelope.
Total fuel avail in the aux's is 86 gallons
Quite so .. but not relevant to the calculation.
Usable is 80 USG. Unusable is included in the empty weight and (although some of it may be usable - ie burnable - in reality) is not factored into weight and balance or routine operational calculations.
Best go back and re-read up on CAO 100.7 and CAO 20.16.
where did you get the 86 gallon figure from Jas. pretty sure it is 80?
For the calculation, you're correct.