C# ekvations problem!

if (bignum > 200)
                {
                    a = bignum / 200;
                    while ((int)(200 - (bignum / a)) < 0)
                    {
                        a += 0.1;
                    }
                }
                else a = 200 / bignum;

 else
                    {
                        g.FillRectangle(bitbrush, (rad * 40), (int)(200 - (calculated[rad] / a)), 20, 2000); //Detta behöver jag hjälp med!
                    }

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;

namespace Inlämningsuppgift_8_del_3
{
    public partial class Form1 : Form
    {
        int[] calculated = new int[7];
        int slag, inmatat;
        Random randomtal = new Random();
        Graphics g;
        Bitmap bm = new Bitmap(300, 200);
        SolidBrush bitbrush = new SolidBrush(Color.Purple); //pensel
        bool randomizedone = false;
        bool drawbm = false;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e) //"slår tärning" random, och sparar resultat i calculated[slag]
        {
            if (drawbm)
            {
                g.Clear(Color.White);
                pictureBox1.Image = bm;
            }
            Array.Clear(calculated, 1, 6);
            textBox1.Clear();
            if (textBox2.TextLength > 0)
            {
                inmatat = Convert.ToInt32(textBox2.Text);

                for (int x = 1; x <= inmatat; x++)
                {
                    slag = randomtal.Next(1, 7);
                    calculated[slag]++;
                }

                for (int i = 1; i < 7; i++)
                {
                    textBox1.Text += calculated[i] + " st " + i + ":or" + Environment.NewLine;
                    switch (i)
                    {
                        case 1:
                            label2.Text = Convert.ToString(calculated[i]);
                            break;
                        case 2:
                            label3.Text = Convert.ToString(calculated[i]);
                            break;
                        case 3:
                            label4.Text = Convert.ToString(calculated[i]);
                            break;
                        case 4:
                            label5.Text = Convert.ToString(calculated[i]);
                            break;
                        case 5:
                            label6.Text = Convert.ToString(calculated[i]);
                            break;
                        case 6:
                            label7.Text = Convert.ToString(calculated[i]);
                            break;
                    }
                }
                randomizedone = true;
            }
            else MessageBox.Show("You have to insert a number in the black textbox!");
        }

        private void button2_Click(object sender, EventArgs e) //Rita ut diagrammet
        {
            drawbm = true;
            if (randomizedone)
            {
                g = Graphics.FromImage(bm);
                int bignum = calculated[1];
                double a;
                for (int biggest = 1; biggest < 6; biggest++)
                {
                    if (bignum < calculated[biggest + 1])
                    {
                        bignum = calculated[biggest + 1];
                    }
                }
                if (textBox2.Text.Length <= 3)
                {
                    a = 200 / bignum;
                }
                else a = bignum / 200;


                for (int rad = 1; rad < 7; rad++)
                {
                    if (textBox2.Text.Length <= 3)
                    {
                        g.FillRectangle(bitbrush, (rad * 40), (200 - (calculated[rad] * (int)a)), 20, 2000); // x, y, width, height
                    }
                    else
                    {
                        g.FillRectangle(bitbrush, (rad * 40), (200 - (calculated[rad] / (int)a)), 20, 2000); //Detta behöver jag hjälp med!
                    }
                }
                pictureBox1.Image = bm;
            }
            else MessageBox.Show("You have to randomize first!");


        }
    }
}