find threshold in histogram

I have a structure the reuslt of hough table, with 1200 values like theta, rho etc.

I made a projection in theta values to show the distribution of the theta.

So i puted on X-axis the agle range from -90:90 and ploted in histogram. I was expecting to see two gaussian curves with negative and possitive theta.

The result is this:

I want to find the threshold between the two curves, i suppose its near -10

I tried Otsu threshold with graythresh function, but it gives a normalized value equals to 0,453, because otsu gives result in [0,1] range.

I wanted to try the the function normalize but my matlab is old.

Any idea how to find the threshold?