"IF" calculations in Adobe Pro forms

I created a form in Adobe Pro and it is working fine except I would like one cell to generate another cells data.  I am use to doing this in excel and using the "if" formula.  When a user enters data in one cell I would like it to generate a 1 in an adjacent cell so that I can use it to  calculate the number of rows used on the form which multiplies it times a rate.  Now I have it set up and it works IF the user remembers to put a 1 in that cell which is left blank on occasion which creates calculation/total problems.