Reading up on the ternary plot, I came to the following conclusion.
(Lets say that the percentage values run as follows: A horizontally at the base from right to left, B from lower left to top and C from top to lower right.) A, B and C are referring both to the percentages and the directions the percentages are plotted here
1) You need only two of the three values to locate the spot, for the third value C will always be C = 100-A-B.
2) This being the case, you could always start from the same corner (which would be 0,0) and use two vectors to calculate the position, thus
3) starting from lower right corner, move the amount of A*S (S = the length of the base or side) of the diagram to the left, then
4) move B% along the direction of side B, the formula for such move being x = S*B*cos(60) and y = S*B*sin(60)
Thus, from the origin point at the bottom right corner, the dot position should be at
x = -A*S + S*B*Math.cos(60)
y = S*B*Mat.sin(60)
That is, if I got that right. I didn’t test my theory.