'Billboard' effect with a Quad

Graphics3D 640,480,32,0
SetBuffer BackBuffer()

Type character
	Field sprite
	Field rot#
End Type

pivot=CreatePivot()
camera=CreateCamera(pivot)

PositionEntity camera,0,3,-5 : RotateEntity camera,30,0,0

ground=CreatePoly(-5,5,10,10) : RotateEntity ground,90,0,0 : EntityAlpha ground,0.5 : EntityOrder ground,1

; Create 5 characters
For i=-4 To 4 Step 2
	c.character=New character
	c\sprite=CreatePoly(-0.5,0.5,1,1) : PositionEntity c\sprite,i,0,0
	EntityFX c\sprite,1
	c\rot#=0
Next

sprite=CreateSprite() : PositionEntity sprite,0,0,4

;-------------------
;Main Game Loop     --------------
;-------------------

While Not KeyDown(1)

For a.character=Each character

	PointEntity a\sprite,camera

Next

;Keyboard Control
If KeyDown( 208 )=True Then pitch#=pitch#-1 
If KeyDown( 200 )=True Then pitch#=pitch#+1
If KeyDown( 203 )=True Then yaw#=yaw#-2
If KeyDown( 205 )=True Then yaw#=yaw#+2

RotateEntity pivot,pitch#,yaw#,0

RenderWorld

Flip

Wend

End

;Functions
;------------

Function CreatePoly(xpos#,zpos#,x#,z#) ;Create a backless Square (sprite)
	sprite=CreateMesh()
	he=CreateBrush(255,255,255)
	v=CreateSurface(sprite,he)
	FreeBrush he
	AddVertex (v,xpos#,zpos#,0,      1,0)
	AddVertex (v,xpos#+x#,zpos#,0,   0,0)
	AddVertex (v,xpos#+x#,zpos#-z#,0,0,1)
	AddVertex (v,xpos#,zpos#-z#,0,   1,1)
	AddTriangle(v,2,1,0)
	AddTriangle(v,0,3,2)
	AddTriangle(v,0,1,2)
	AddTriangle(v,2,3,0)
	Return sprite
End Function

I'm trying to achieve the same 'billboarding' effect that Blitz3D automatically gives Sprites, but in a Quad.
The example code shows five of these Quads pointing at the Camera with PointEntity, and a Sprite behind them.
Moving the camera with the cursor keys, it's clear that the Quads aren't acting in the same way as the Sprite.

Does anyone have ideas as to how the correct 'roll' value for each Quad can be calculated?

Thanks.