You cant use ReportItems!Test.Value in a calculated field, thats not possible as well as aggregate functions.
You have to write your calculation in your sorting expression of the tablix. For example:
'Expression of Textbox1 in Tablix1
=Fields!Sales.Value - Fields!Income.Value
'Expression of Textbox2 in Tablix1
=Fields!Sales.Value / Fields!Income.Value
'The next two expressions are the same
=ReportItems!Textbox1 * ReportItems!Textbox2 'Label1
=(Fields!Sales.Value - Fields!Income.Value) * (Fields!Sales.Value / Fields!Income.Value) 'Label2
Go to your tablix under Tablix Properties > Sorting > Add > Expression and write in the expression from Label2. This way you can sort after your expression.
If you have some groupings in your tablix you can also go to your tablix under Row Groups > Group Properties (the drop down) > Sorting > Add > Expression.