We don’t have your data, so I can only outline the approach I would take.
If you calculated the line from the same x and y values of the stress-strain curve, subtract the y value of the line from the y value of the stress strain curve for the x values common to both. The y value of the line corresponding to the x coordinate of the ‘last positive’ value of that vector is the one you want.
This illustrates the approach:
x = 0:0.1:2; % Create Data
y = -(x-5).^2 + 25; % Create Data
b = polyfit(x(1:15),y(1:15),1); % Fit Line
yf = polyval(b,x); % Evaluate Line
dif = y-yf; % Subtract Data From Line
xn = find(dif > 0, 1, 'last'); % Find Index Of Last +ve
figure(1)
plot(x,y) % Plot Data
hold on
plot(x,yf) % Plot Line
plot(x(xn),y(xn),'+r') % Plot Point
hold off
grid
