Finding Angles

Hello. I need to find an angle to the top-left part of my screen from a random point on the screen. I believe that this should work:

Angle = PosY/PosX
Angle = 180-ATan(Angle)

but doesnt :(

The reason for what i have done is that i worked out from anywhere on the screen i have a triangle like:

____
\   |
 \  |
  \x|
   \|

And the opposite and adjacent sides are my x and y co-ords. So i use the Tan-1 funtion to work out angle x which is the angle if you start at the top of the screen anti-clockwise. So as angles start from the bottom of the screen clockwise i get the angle i need (y) by taking the angle away from 180:

   \x|
    \|
    y|
     |

But this doesn't work. I'm not very good at this sort of stuff so there may be something i've done wrong in he calculations.

If there is anyone good at maths out at there that can help me i'd be very grateful. (and i hoped my diagrams helped :))

this might help:
http://homeworktips.about.com/library/weekly/aa120497.htm
http://www.mathstutor.com/Trigonometry(i).html
http://sacademy.cbv.ns.ca/grsrts/protec12.2/math.htm

Basic trigonomotry remember the great Indian cheif SOHCAHTOA:

Sine
Opposite
Hypotinus

Cosine
Adjacent
Hypotinus

Tangent
Opposite
Adjacent

Sin( a ) = Opposite / Hypotinus
Cos( a ) = Adjacent / Hypotinus
Tan( a ) = Opposite / Adjacent

look up trigonomitry stuff on google if i am incomprehensible.

ATan2( y, x ) gives the angle for a vector from 0,0 to x,y.

The assumption is that the X axis is 0° and Y is 90°.

Compared to this you have X,Y swapped: X axis is 90° and Y is 0°.
And you are going the opposite way, from x,y to 0,0.

So you want ATan2( -x, -y ). Or something. Just tinker with it until you get the effect you want.

Ok, thanks for all the help.

Btw, we were taugh it as:

Some Old Hippos
Can Always Have
Tons Of Apples

Here's the Function I use for calculating the angle from one point to another:

Function calcAngle2D( x1, y1, x2, y2 )
	Local angle:Int=ATan2( ( y2 - y1 ), ( x2 - x1 ) ) + 180
	Return angle:Int
End Function

Muttley