Percenage Calculation Acrobat Professional 8

Have a form with two columns. One column has user entered value that sum at bottom. Second Column calculate percent of the total. The percent amount calculates fine when the first column is initially populated, but when the initial value is deleted (like it would be for a user to use), the error message says "value entered does not match the format of the field". Current calculations is:
ContributionRevenue1 / TotalRevenue1

Have tried the following calculations:
If (ContributionRevenue1 != Null)
{ContributionRevenue1 / TotalRevenue1}

and

var f = this.getField("ContributionRevenue1");
var g = this.getField("TotalRevenue1");
event.value = f.value / g.value
and nothing is calculated with either. Anyone have any suggestions.