You should use the Items property. In other words, your variable should start with its value set to whatever it should be before applying the selected items. Then, iterate over the items, applying your logic against the variable one item at a time for each item that is selected. Then, no matter what the user selects or de-selects, you'll always arrive at the proper value for your variable.
int myValue = 0;
foreach(ListItem item in cbl.Items)
{
if(item.Selected) myValue += int.Parse(item.Value);
}
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