How to use if/then for a calculation field

When the Acres field is blank or zero, you're dividing by zero, which results in either + or - Infinity, or the special numeric value of NaN. If the field is formatted as a number, any of these values will cause an error in the formatting routine. What you probably want is if the Acres field is blank, then the result of the calculation is blank as well. You need to use JavaScript for this:

// Custom calculation script

(function () {

    // Get the field values, as numbers

    var waf = +getField("WAF1").value;

    var acres = +getField("ACRES1").value;

    // Set this field's value

    if (acres !== ) {

        event.value = waf / 12 / acres;

    } else {

        event.value = "";

    }

})();

Since you have multiple rows, you should make this into a document-level function so it can be called from the calculation script of each WAI field. It would make things easier if you changed the field names so there is a separator character between the field name and number, like: WAF_1 and ACRES_1

This makes it easier to get the row number from the field name. Post again if you need more help.