ITaxable "Tax" changes now update ExTax, rather than IncTax values

InABox.Core/Entity.cs

@@ -263,7 +263,7 @@ namespace InABox.Core
 
                     else if (name.Equals("Tax"))
                     {
-                        taxable.IncTax = (double)after + taxable.Tax;
+                        taxable.ExTax = taxable.IncTax - (double)after;
                     }
 
                     else if (name.Equals("IncTax"))