Help keeping a calculated text field blank until all conditions are met

I have a form in which the latitude and longitude for a starting point can be entered, followed by the latitude and longitude for an ending point. The distance between the two coordinates is then automatically calculated. As it is right now, If I enter a value in the first latitude column and the first longitude column, the distance column will populate with a value, even though the second latitude and longitude columns are blank. I would like the distance column to remain blank until a complete data set has been entered. If anyone could help me with this, I'd surely appreciate it.  

Script for the distance text field of the first row:

function distance(lat1, lon1, lat2, lon2, unit) {
    var radlat1 = Math.PI * lat1/180
    var radlat2 = Math.PI * lat2/180
    var theta = lon1-lon2
    var radtheta = Math.PI * theta/180
    var dist = Math.sin(radlat1) * Math.sin(radlat2) + Math.cos(radlat1) * Math.cos(radlat2) * Math.cos(radtheta);
    dist = Math.acos(dist)
    dist = dist * 180/Math.PI
    dist = dist * 60 * 1.1515
    if (unit=="K") { dist = dist * 1.609344 }
    if (unit=="N") { dist = dist * 0.8684 }
    return dist
}

var lat1 = this.getField("Alat1.0").value
var lon1 = this.getField("Alon1.0").value
var lat2 = this.getField("Alat2.0").value
var lon2 = this.getField("Alon2.0").value
//Assuming Miles
this.getField("rel1.0").value = distance(lat1, lon1, lat2, lon2)