Trig troubles with camera.

I'm trying to lock my camera so it doesn't drop below 0 height. I use 2 pivots for controlling rotation. I'm using the Mouse_lib.bb for reading mouse. Here's my code...

rostrum_pan=CreatePivot()
rostrum_tilt=CreatePivot(rostrum_pan)
camera=CreateCamera(rostrum_tilt)
CameraZoom camera,CAMERA_ZOOM
PositionEntity camera,0,0,-300
RotateEntity rostrum_tilt,35,0,0
PositionEntity rostrum_pan,0,50,0

mouse_z=mouse\speedz*10*modifier
test=EntityZ(camera)+mouse_z
If Abs(test)<3500
	MoveEntity camera,0,0,mouse_z
EndIf

If mouse\br\down
	TurnEntity rostrum_pan,0,mouse\speedx / 5.0,0
	TurnEntity rostrum_tilt,mouse\speedy / 5.0,0,0
	mouse\x=mx
	mouse\y=my
	MoveMouse mx,my
EndIf	
If EntityY(camera,1)<0			; If camera under 0 y
	theta#=ACos(EntityY(rostrum_tilt,1)/Abs(EntityZ(camera)))
	RotateEntity rostrum_tilt,theta,0,0,1
EndIf

Trouble is, it don't work!