Can you post the formula? As txnelson said you can simply make the checking part of the formula.
To post the formula:
1. Open your dataset and click on the + sign next to the formula column.
2. Double-click inside the formula window. This will change the window to show the JSL code for the formula
3. Copy the JSL code and post it to this forum.
The new formula will be something like this:
tmp = put-original-formula-here;
if (tmp < 0, 0, tmp);