Try/Catch Statements

[Zofu]

Hello there! I am fairly new to C# and I'm having a problem coding a specific assignment. I was wondering if anyone could provide a few tips on what I'm doing wrong. I'm trying to have it catch 3 different things. That there is a decimal value, that there actually is something inserted, and that the number is within range of 0 and 1000. All of these need to display a different message box also. Down below is so far what I've done. I'm able to do the valid range but it's still calculating the numbers when I click calculate. Aslo when there is no value entered it brings up the "Please enter a valid subtotal" message box instead of the "Requires Subtotal" box. Thanks!

private void btnCalculate_Click(object sender, EventArgs e)
        {
            try
            {
                
                
                decimal subtotal = Decimal.Parse(txtSubtotal.Text);
                if (subtotal < 0 || subtotal > 1000)
                {
                    MessageBox.Show("Please enter a valid range");
                    txtSubtotal.Focus();
                                    }
                if (txtSubtotal.Text == "")
                {
                    MessageBox.Show("Requires Subtotal");
                    return false;
                }
                
                           decimal discountPercent = .25m;
                decimal discountAmount = subtotal * discountPercent;
                decimal invoiceTotal = subtotal - discountAmount;

                discountAmount = Math.Round(discountAmount, 2);
                invoiceTotal = Math.Round(invoiceTotal, 2);

                txtDiscountPercent.Text = discountPercent.ToString("p1");
                txtDiscountAmount.Text = discountAmount.ToString();
                txtTotal.Text = invoiceTotal.ToString();

                txtSubtotal.Focus();

                Convert.ToDecimal(txtSubtotal.Text);      
            }
            catch (FormatException)
            {
                MessageBox.Show("Please enter a valid subtotal");
            }
           
        }