Finding normal curvature from principal curvatures?

I have two principal curvatures defined as:

k1 = H + sqrt(H.^2 - K);

k2 = H - sqrt(H.^2 - K);

and I would like to define the normal curvature as:

kn = k1.*cos(the).^2 + k2.*sin(the).^2;

However, I'm having some trouble finding the (theta). What I understand is that this will be the angle in the tangent plane measured CCW from the direction of minimum curvature. My principal curvatures are saved as an [x,y] matrix of values -- how do I find the direction of minimum curvature and relate this back to the tangent plane?