Let's say that your curve is described by the vectors x and y (I imagine that you obtain that curve via plot(x,y)).
Then you can find the maximum of the curve as:
y_max = max(y);
You can then pick the indices of the points corresponding to 10% and 90% of the maximum as:
idx_y10 = find(y == 0.1*y_max);
idx_y90 = find(y == 0.9*y_max);
The values that you are searching on the x axis will be:
x_10 = x(idx_y10);
x_90 = x(idx_y90);
I did not try it, enjoy!