As the (I guess) the formula of such ellipses should be:
with x as time and y the price you have to know A B D E and F and you have to resolve it to x: x = +/-Sqrt(...).
Then you can easily get the two prices at a specified time.
Or you loop at a specified time through the prices between CHART_PRIOCE_MAX and CHART_PRIOCE_MIN until:
Ax²+Bxy+Cy²+Dx+Ey+F < epsilon
with epsilon = 0.5*Point.