Here's the approach I'd take; I don't have access to the software right now.
1 hidden text field is hooked up to your data source, 1 hidden text field is your max value (default to 0).
Add an event listener to the hidden text field hooked up to the data source OnSetText; from there, you'll grab the other text field, compare the max value field and the updated current value passed to the OnSetText function, if it's larger, make the text of the max value text field the new value. Otherwise you do nothing.
There may be a more elegant solution using Visual Logic, but it's a fairly simple thing in scripting too.
#XPression