angle would be arctan(dy/dx). dy = 12-300 = -288. dx = 250-100=150.
dy/dx is the slope between the two points, or -288/150 pixels.
take the arctan of that, and you get about -62.488 degrees. We double check to make sure the quadrants are right (otherwise we would have to throw in 180 degrees).
arctan in mIRC is $atan, but it uses radians. To remember how to go back and forth between degrees and radians, remember that a circle is 360 degrees and 2*pi radians. so 360 degrees = 2*pi radians, or 180 degrees = pi radians. For the first 1.24 trillion places of pi, find this guy
for reference, 0 degrees would be to the right of the first point.