I understand that programming is your hobby, but it does not mean that you shouldn't improve your skills. To start with I strongly suggest using better naming rules (prefixes for controls, meaningful variable names etc.).
private void GoButt_Click(object sender, RoutedEventArgs e)
{
double VL = 0;
double VA = 0;
int idxGSL = GSL.SelectedIndex;
int idxCSL = CSL.SelectedIndex;
if (!Double.TryParse(LO.Text, out VL))
{
return;
}
if (!Double.TryParse(AO.Text, out VA))
{
return;
}
if (idxGSL == -1 || idxCSL == -1)
{
return;
}
double VTS = VL * VA;
double tmp = (((idxCSL + 1) * VTS) / (32 + 4 * idxGSL));
TSF.Text = VTS.ToString();
TMS.Text = (VL * 12 / 16).ToString();
TMT.Text = (VL * 2 / 10).ToString();
TMSc.Text = (VL * 12 / 16 * 4).ToString();
TGBS.Text = (VTS * 1.5 / 258).ToString();
TGB.Text = Math.Ceiling(tmp).ToString();
}