C# WinApp มีข้อสงสัยเกี่ยวกับตัวแปรที่เป็น double

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        
        int[] pipett = new int[] { 1,2,3,4,5,6,7,8,9,10,25 };
        int flask = 100;
        #region _Function
        string stepcalSTD(double conc)
        {
            string str = "";
            double cri = 1000;
            int pp = 0;
            int i = 1;
            do
            {
                
                if (cri / conc > 99)
                {
                    str += Environment.NewLine + "Step " + i + " เตรียมความเข้มข้น " + cri / 10 + " mg/L  ปิเปต 10 ml จาก " + cri + " mg/L ปรับ 100 ml";
                    cri = cri / 10;
                }
                else
                {
                    pp =  _Pipett(cri, conc);
                    str += Environment.NewLine + "Step " + i + " เตรียมความเข้มข้น " + conc + " mg/L  ปิเปต " + pp + " ml จาก " + cri + " mg/L ปรับ 100 ml";
                    break;
                }
                System.Threading.Thread.Sleep(10);
                i++;
            } while (cri * pp / 100 != conc);
            return  str;
        }
        int _Pipett(double _C1 = 0, double _C2 = 0)
        {

            int p = 0;
            double cc = 0;
            foreach(int i in pipett)
            {
                cc = Convert.ToDouble(_C1 * i / flask);
                MessageBox.Show(_C1 + " " + i + " " + cc + "  " + _C2);
                if (cc == _C2)
                {
                    p = i;
                    return p;
                }
                System.Threading.Thread.Sleep(10);
            }
                return p;
        }
        #endregion
        private void button1_Click(object sender, EventArgs e)
        {

            textBox2.Text = "";
            double startCounter = DateTime.Now.TimeOfDay.TotalSeconds;
           
            if (double.Parse(textBox1.Text) == 0) return;
            this.textBox2.Text = stepcalSTD(double.Parse(textBox1.Text));
            this.textBox2.Text += Environment.NewLine;
            this.textBox2.Text += Environment.NewLine + string.Format("Total time: {0} seconds", (DateTime.Now.TimeOfDay.TotalSeconds - startCounter).ToString("#,##0.0###"));
        }
    }