this piece of code will give you the angle . Demo based on a half sinus wave.
you can easily apply it to your case
y = sin(x);
minus3dBpoint = 1/sqrt(2);
z = abs(y-minus3dBpoint);
[p,loc] = findpeaks(-z);
angle = diff(x(loc))/2
of course , my example (sinus) has a peak amplitude of 1 , which is not your case
you'll figure out how to adapt the code....