You can't do any script triggers natively from filemaker 8.5. You'd need to either upgrade or perhaps you can find a plug in for this.
Even in filemaker 10 and 11, you could at best write a script that checks the value of the field and responds accordingly when the user exits or saves a field or record.
Not sure if you can do that directly, but you could certainly use other action triggers such as committing the record to run a script of the type:
If (YourTable::YourCalcField = 12345)
Perform Script (YourScript)
That would amount to the same thing (if you trigger this script on commit on each layout based on each table where the components of your calculation reside). You could go additionally trigger this on layout load and OnModify on the pertinent fields as well if there are still loopholes in the OnCommit trigger.
Being a simple If, it shouldn't slow you down very much...but of course that depends on the complexity of the calculation in question. It would be recalculated constantly...
Ooops. Missed the "8.5" part. Sorry. Thanks Phil!