Not positive but check the rainpermutations file in the Render/shaders200a/game
At the start there is:
float2 CoordArray[] = {
float2(-ONE_STEP,0),
float2( ONE_STEP,0),
float2(0, ONE_STEP),
float2(0,-ONE_STEP),
float2( ONE_STEP, ONE_STEP),
float2(-ONE_STEP,-ONE_STEP),
float2(-ONE_STEP, ONE_STEP),
float2( ONE_STEP,-ONE_STEP),
float2(-ONE_STEP_X2,0),
float2( ONE_STEP_X2,0),
float2(0, ONE_STEP_X2),
float2(0,-ONE_STEP_X2),
float2( ONE_STEP_X2, ONE_STEP_X2),
float2(-ONE_STEP_X2,-ONE_STEP_X2),
float2(-ONE_STEP_X2, ONE_STEP_X2),
float2( ONE_STEP_X2,-ONE_STEP_X2)
};
There are other rainpermutations files but I believe this is the most recent and the one to change.
I have not ventured farther than that. I am sure there are some more variables you can find in that file that will help.