Hint --


Let k be the fraction we are trying to find when CD = 0.8(FE). Then CG is 0.4(FE). Let

GE = k(FE)

FG = (1 - k)(FE)

(k - 0.2)(k - 0.8) = 0

Therefore the two possible values for k are 0.2 or 0.8.  Each would leave a crease of length 80 % of the diameter, but the larger would be a notch nearly through the tree. The acceptable value of the depth of the notch is 0.2 or of the diameter.