Largura * Complemento = Subtotal + Porcentagem = Total
subtotal.Text = (Convert.ToDouble(largura.Text) * Convert.ToDouble(complemento.Text)).ToString();
total.Text = (Convert.ToDouble(subtotal.Text) % Convert.ToDouble(PercaTextBox3.Text)).ToString();
Where am I missing the percentage?
The percentage symbol does not calculate the percentage, you have to use basic math to do this. Alias I suggest learning to program in a more structured way. Learning in trial and error without knowing what you are doing is more difficult, even if it does not seem like it, it spends more time, skips important things, and mostly teaches wrong causing tragedies.
You have an extra error in your code that I will take to resolve. The conversion of data coming externally is not guaranteed and can break your application, you have to make sure that something has been typed that can be converted.
I did it in Console because it is easier to demonstrate, but you adapt to Windows Forms. I even made one last conversion to string that will only make sense in Windows Forms.
It has meaningless conversions in the code. So reinforcing to understand what you are doing, in the current way is not learning to program. A phrase I always say:
If you do not know everything your code does, including white space, you still can not program
using static System.Console;
public class Program {
public static void Main() {
var larguraText = "2.00";
var alturaText = "1.00";
var percText = "30";
var total = 0.0;
if (double.TryParse(larguraText, out var largura) && double.TryParse(alturaText, out var altura) && double.TryParse(percText, out var perc)) {
total = largura * altura * (perc / 100);
}
var totalText = total.ToString(); //só para fins didáticos do exemplo
WriteLine(totalText);
}
}
See running on .NET Fiddle . And in Coding Ground . Also I placed GitHub for future reference .