How do I populate field 1 with the amount from field 2 based on the description of field 3?

For that to work, the custom calculation script for field 1 would have to be something like:

// Get the field values

var sDesc = getField("description").valueAsString;  // string

var nAmount = getField("amount").value;  // number

// Set this field value

if (sDesc === "Single Life") {

    event.value = nAmount;

} else {

    event.value = "";  // Blank this field

}

You can add "else if" blocks to handle other cases, or use the "switch" JavaScript statement instead, which is what I'd recommend.

As the custom calculation script of field 1 you can use this code, for example:

if (this.getField("description").valueAsString=="Single Life") event.value = this.getField("amount").value;

else event.value = "";