Calculation Field to be "0" until data entered

That's possible using a custom calculation script, something like:

// Custom calculatino script for field C

(function () {

    // Get the field values

    var A = +getField("A").value;  // Get value as a number

    var B = getField("B").valueAsString;  // Get the value as a string so we can tell of it's blank

    // Set this field's value to the sum if there's a value in field B

    event.value = B ? A + +B : "";

})();

That last line can be translated to English as: If there is an entry in the B field (B ?), then convert the string B to a number (+B) and add it to the A value (A + +B) and set this field's value to the sum (event.value =). Otherwise set this field to blank (: "").

It's equivalent to:

if (B !== "") {

    event.value = A + +B;

} else {

    event.value = "";

}

just more concise.