Adding lookup field to the bootom click and next add item list

Question

If I add a element to list:

                protected void Button2_Click(object sender, EventArgs e)
            {
                using (SPWeb witryna = SPContext.Current.Web)
                {
                    SPListItemCollection listItems = witryna.Lists["Szczegoly"].Items;

                    SPListItem item = listItems.Add();
                    item["Kontrakt"] = TextBox1.Text;
                    item["Pracownik"] = TextBox2.Text;
                    item["Dzien"] = TextBox3.Text;
                    SPFieldLookupValue splkup = new SPFieldLookupValue(5,TextBox4.Text);
                    item["Procenty"] = splkup;
                    item.Update();
                }
            }

A lookup fields adding in item called "Procenty" a "3/8" percent value

I', getting only 3/8 value not 5/8. How o get get a IDis in this function?