在pictureBox中画网格出现的问题

用以下代码画网格，可是显示出的线段不能对齐，甚至横线在竖线右部的外边缘还冒出了一段，不知何故。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Drawing.Drawing2D;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
                                          
            Graphics g = pictureBox1.CreateGraphics();
            Pen bluePen = new Pen(Color.Blue, 3);

            for (int i = 0; i < this.pictureBox1.Width; i++)
            {
               ;
                g.DrawLine(bluePen, new Point(i, 0), new Point(i, this.pictureBox1.Height));
                i += 60;
            }
            //水平
            for (int j = 0; j < this.pictureBox1.Height; j++)
            {
                g.DrawLine(bluePen, new Point(0, j), new Point(this.pictureBox1.Width, j));
                j += (this.pictureBox1.Height - 6) / 10;
            }

        }
    }
}