Hi, does anybody know how can I reduce or change the area while using Dist()  ?

I'm familiar with tuts from fun programming, but I'm new here... 

1. void setup(){
   frameRate (10);
   size (1000, 700);
   background (0);
   rectMode (CENTER);
   }
   
   
   
   void draw(){
   for (int x = 25; x <= 1000; x +=50) {
   for (int y = 25; y <= 700; y +=50) {
   float d = dist (x, y, mouseX, mouseY);
   rect (x, y, 50, 50);
   fill (255-d, 0, 0);
   }
   }
   }