I'm sorry, but that is not a very clear question. It sounds like you are getting a divide by 0 error, and instead you want the result to be 1 (or 100%, depending on your display format)
Are you using Access?
Is the calculation done in a query?
If so to both, you can use the Iif and IsError functions to solve your problem
Something like
YourFieldName:Iif(iserror(YourCalculation),1,YourCalculation)
Kathryn