p5_grid.js

// set up grid in p5
var rows;
var cols;
var cell_w = 72;
var cell_h = 77;

function setup() {
  createCanvas(500, 500);
  background(0);

  // set up grid
  cols = Math.floor(width / cell_w);
  rows = Math.floor(height / cell_h);
  
  // define margins
  offset_x = (width % cell_w) / 2;
  offset_y = (height % cell_h) / 2;
  
}

function draw() {
  noFill();
  stroke(255);
  for (var i = 0; i < rows; i++) {
    for (var j = 0; j < cols; j++) {
      push();
      translate(j*cell_w + offset_x, i*cell_h + offset_y); // translate to upper left corner of each cell instead of calculating the coordinates
      rect(0, 0, cell_w, cell_h);
      pop();
    }
  }
}