Angle calculation with camera tilted downwards

I followed this tutorial to calculate joint angle of left knee but this takes into consideration when the sensor is placed straight (I have asked the developer of this blog). I have to tilt the camera downwards to see feet and for that the camera tilt should also be included in angle calculation. As the camera is rotated along X axis, the rotation matrix will be:

x: [1, 0, 0]

y: [0, cos A, -sin A]

z: [0, sin A, cos A]

where A is the camera angle tilted downwards (clockwise rotation)

I calculated camera angle as follows:

cameraAngleRadians= Math.Atan(floorClipPlane.Z / floorClipPlane.Y);

Now how should I use this rotation matrix in following formula:

angle formula

Any help will be really appreciated. Hoping to get response soon. I am using Unity to program and sensor used is Orbbec Astra Pro