You go it in the title
Code:

void setup() {
  size(400, 400);
}

void draw() {
  float xpos = mouseX - width * 0.5;
  float ypos = mouseY - height * 0.5;
  println(xpos + " | " + ypos);
  
  line(width * 0.5, 0, width * 0.5, height);
  line(0, height * 0.5, width, height * 0.5);
} 

Hope that helps
rS

Also have a look at map(), which will give you a more general way to control what the range of values is you want.

eg: float x = map(mouseX, 0, width - 1, MIN_X, MAX_X);

That is, a mouseX value of 0 is mapped to MIN_X, a value of (width - 1), which is the right-most pixel location, is mapped to MAX_X, and values in between are scaled smoothly between MIN_X and MAX_X. Incidentally, values outside of the range specified by the 2nd and 3rd parameters will also be mapped linearly, but will appear outside the range specified by the last two parameters.

-spxl