VAT Calculation Problem

Question

Guys, I have a formular for calculating VAT it's most likely not the best but it works. My problem is now that I have to add a value from damage_WaiverTextBox.Text to the final total_ChargeTextBox.Text as the value in damage_WaiverTextBox is zero rated VAT and just wants to be added to the total. I Hope I make sense.
The current code is:
decimal subtotal = 0.00M,
VATRate = 0.00M,
VAT,
Total;            try
{
subtotal = decimal.Parse(subtotalTextBox.Text);
}
catch (FormatException)
{
MessageBox.Show("The value you entered for the " +
"Input Amount is not valid");
}             try
{
VATRate = decimal.Parse(vATrateTextBox.Text);
}
catch (FormatException)
{
MessageBox.Show("Wrong Value: The VAT rate must " +
"be a value between 0 and 100");
}             decimal I = VATRate / 100;
VAT = subtotal * I;
Total = subtotal + VAT;
vATTextBox.Text = VAT.ToString("F2");
total_ChargeTextBox.Text = total_ChargeTextBox.Text = Total.ToString("F2");

Vulpes · Accepted Answer

Yes, if that's the only zero-rated item, you could do:
decimal damageWaiver = 0m;
try    {        damageWaiver = decimal.Parse(damage_WaiverTextBox.Text);    }    catch (FormatException)    {       MessageBox.Show("The value you entered for the " + "Damage Waiver Amount is not valid");    }
Total += damageWaiver; total_ChargeTextBox.Text = Total.ToString("F2");

mike Delvotti · Answer

Thank you Vulpes - That's perfect. I was going round in circles.

mike Delvotti · Answer

Hi Vulpes, I see your point. Basiclly on the form there are many textboxes that all add together and total in the subtotal textbox then VAT is applied at the rate that is in VATtextbox i.e 20% and the final total displays in total_ChargeTextbox.
Damage_waiver is a fixed amount and doesn't total in the subtotal textbox but needs to be applied to the final calculation after VAT has been applied in total_chargeTextbox
It would be great if the value in Damage_waiverTextbox could just be applied to the total_chargeTextbox as the demo below:
decimal I = VATRate / 100;
VAT = subtotal * I;
Total = subtotal + VAT;
vATTextBox.Text = VAT.ToString("F2");
total_ChargeTextBox.Text = total_ChargeTextBox.Text = Total.ToString("F2");
//Now add the damage waiver amount to the final total
total_ChargeTextBox.Text = total_ChargeTextBox.Text +Damage_WaiverTextbox.ToString("F2");
The above doesn't work as it doesn't sum it just adds it on the end of the output.
I was thinking that by adding the net amount in Damage_waiver to the total amount (inc VAT) would do the job or am I way of the mark? :)

Vulpes · Answer

Mike,
Unless you know in advance what the VAT rates will be for each textbox, it seems to me that you will need a VAT rate box for each one if you're going to total the net amounts, VAT thereon and gross amounts for all the textboxes.
As far as I can tell, you only have one VAT rate textbox here.