Further thought:
The "actual" value could be maintained in the totfailrate variable, if it is a module wide variable, but..
If user is expected to enter a value (or accept the calculated value), in which case, display/actual value gymnastics are needed.

But, if the box is only to display the calculated value (in the desired format), using a Label, rather than a TextBox, would discourage the user from trying to change a value that they shouldn't.