Setalpha by distance traveled

Alright here's an example i knocked up, just paste 'n compile.

click to create a partical explosion with 50 particals and a max distance of 100.

But as you can see it doesnt work correctly, what am i doing wrong?

SeedRnd MilliSecs()

Global ParticalList:TList = CreateList()


Type TPartical
	Field x:Int,y:Int,a:Float,r:Int
	Field ox:Int,oy:Int
	Field img:TImage
	Field dx:Int,dy:Int
	Field maxdistance:Float
	Field fade:Int
	
	Method Create:TPartical(_x:Int, _y:Int, _img:TImage, _dx:Int = 0, _dy:Int = 0, mxdist:Float = 0, _fade:Int = False)
		ListAddLast(ParticalList,Self)
		x = _x
		y = _y
		ox = _x
		oy = _y
		img = _img
		dx = _dx
		dy = _dy
		If mxdist
			maxdistance = mxdist
		End If
		If _fade
			fade = True
		End If
	End Method
	
	Method Update:TPartical()
		x:+dx
		y:+dy
		SetRotation(r)
		SetAlpha(a)
		DrawImage img,x,y
		SetRotation(0)
		SetAlpha(1)
		
		If maxdistance
			If DistanceTraveled() >= maxdistance
				Destroy()
			End If
		End If
		
		If fade
			a = DistanceTraveled()/maxdistance
		End If
	End Method
	
	Method SetPAlpha(alpha:Float)
		a = alpha
	End Method
	
	Method SetPRotation(rot:Int)
		r = rot
	End Method
	
	Method Destroy()
		ParticalList.Remove(Self)
	End Method
	
	Method SetX(_x:Int)
		x = _x
	End Method
	
	Method SetY(_y:Int)
		y = _y
	End Method
	
	Method DistanceTraveled:Float()
		Local returnx:Float=Abs(ox-y)
		Local returny:Float=Abs(oy-y)
		Return returnx+returny	
	End Method
	
	
End Type


'Partical explosion function; creates a explosion of particals
Function ParticalExplosion(_x:Int, _y:Int, image:TImage, n:Int, dist:Float)
	Local dx:Int = Rand(-3,3)
	Local dy:Int = Rand(-3,3)
	
	For Local i:Int = 1 To n
		Local part:TPartical = New TPartical.Create(_x, _y, image, dx, dy, dist, True)
	Next
	
End Function

AppTitle = "Partical test - click to fire"
Graphics 800,600

'Create a dot image (our partical)
Global dot:TImage = CreateImage(5, 5) 
DrawOval 0, 0, 5, 5
GrabImage(dot,0,0)

While Not KeyHit(KEY_ESCAPE)
Cls
	For Local UpdateParticals:TPartical = EachIn ParticalList
		UpdateParticals.Update()
	Next
	If MouseHit(1) ParticalExplosion(MouseX(), MouseY(), dot, 50, 100)
Flip
Wend
End