checkerboard effect questions

//checkerboard effect
 
precision lowp float;
varying vec2 vTex;
uniform sampler2D samplerFront;
uniform float pixelWidth;
uniform float pixelHeight;
 
//params
uniform float squareSize;   //size of checkerboard squares in pixels
uniform float strength;     //amount of lightening and darkening to apply
 
void main() {
    vec2 screenSize     = vec2(1.0/pixelWidth, 1.0/pixelHeight);
    vec2 grid           = floor((vTex * screenSize) / squareSize);  //checkerboard grid coordinate, 0,0 is top left square
 
    float checkerboard  = mod(grid.x + grid.y, 2.0)-0.5;            //-0.5 for dark +0.5 for light
 
    vec4 front          = texture2D(samplerFront, vTex);
    front.rgb           += checkerboard * strength;
    gl_FragColor        = front * front.a;
}[/code:2frdzt6h]

Hi all, I made this checkerboard effect for a selection highlight for an in-game level editor. It started as a modification of @chrisbrobs colorcheck effect so thanks to him [url=https://www.scirra.com/forum/effect-colorcheck_t163486]effect-colorcheck_t163486[/url]

The differences are that it's no longer proportional to screen size (parameter is 'squaresize' rather than 'squarecount'), it now adds and subtracts the source pixels with a strength parameter rather than overriding them completely, and I cut out the colorization because I did not need it.

This was my first effect and I have some questions:

[b]Is there an easier way of getting absolute screen-space pixel coordinates? [/b]
Having to calculate the screens size from 1/pixelWidth and 1/pixelHeight, and then multiplying that to the vTex coordinates seems wrong. 

[b]If void main is called for every pixel, is it unnecessary to put certain calculations in there?[/b] 
For example, the screen size calculation above should only need to be done once. Other shaders seem to do things like this as well, is this because the performance hit is negligible?

[b]Why isn't alpha reserved when I only adjust the rgb? [/b]
On the last line I multiply front by front.a to ensure the transparency of the sprite is reserved, but I am not sure why this is necessary:
front is a vec4, sampling the RGBA of the pixel
front.rgb is then adjusted to apply the checkerboarding 
but front.rgb is only the first 3 values in the vec4, shouldn't the front.a be left intact?

thanks

void main() {
	vec2 testSize		= destEnd - destStart;
	gl_FragColor		= vec4(vTex.x ,vTex.x, vTex.x, 1);
}[/code:opumu9fb]

[url=http://i.imgur.com/PdERKrZ.png]http://i.imgur.com/PdERKrZ.png[/url]

This image shows how with the line above, the vTex is relative to the screen but with the line above disabled, the vTex becomes local to each sprite.