How to get the shortest way off screen?

Okay, I must be more dumb or tired than I think I am, because I can't quite figure out how to do this...and I usually don't have problems with SIN and COS.
This problem needs to calculate distance though, and that probably needs SQR and I'm not so strong there.

What I need is an easy way to calculate the angle of the shortest distance from the cursor to any edge of the screen.

The following example doesn't do what I want, but it's a framework.
Can someone help me with the code needed to calculate the angle to draw the shortest possible line, from the cursor to the edge of the screen?

Graphics 640,480,16,2

Repeat
Cls

mx=MouseX():my=MouseY()

For angle=0 To 360 Step 12; Speed is more important than precision.

	tox = mx+Sin(angle) * 200;Just a random number
	toy = my-Cos(angle) * 200
	Line mx,my,tox,toy

Next

Flip
Until KeyHit(1)

Thanks.

The shortest way off the screen will always be a horizontal or vertical line.

So, given Width and Height equal to the width and height of the screen...

Lowest = Mx
ToX = 0
ToY = My

If My < Lowest
   Lowest = My
   ToX = Mx
   ToY = 0
EndIf

If Width-Mx < Lowest
   Lowest = Width-Mx
   ToX = Width
   ToY = My
EndIf

If Height-My < Lowest
   ToX = Mx
   ToY = Height
EndIf 

D'oh! - that makes sense.

There you see - sometimes you can just overcomplicate things by thinking too much...so I'll try not to think too much in the future. :-)

Thank you.

Have a nice weekend. :-)