The variable, although it turns into a value, will recalculate every time you reload, so this may be fine for your purposes.
Alternatively, you can put quotations around your expression to have it passed in as an expression, rather than a value.
E.g. Expression = 'max(x)'
This page provides an explanation of this concept https://help.qlik.com/en-US/sense/1.1/Subsystems/Hub/Content/Scripting/ScriptVariables.htm