Multiple things come to mind:
- Is using a View a possibility, as that will be dynamic?
- What about a Trigger, whenever any of the two fields are updated...the third field should be updated automatically.
- How many records are involved and frequency of change? Is it possible to run the calculate everyday (whatever suits your need) and then use the updated data? Probably can be done from the Database end too...
Have you already tried something and observed any issues?
On a side note: The question in the thread, need not be the entire question in itself. It should a small synopsis probably to give an idea. A lot of people ignore questions\threads, which are not formatted properly.