Setting default value to a blank field

Hi, wondering if someone could help me. I have looked high and low for answers in forums and turorials etc.

I'm creating an automated Invoice in acrobat 9 on mac.

I'm after a solution to make automated fields in the totals collumn (please see screenshot attached) to have a default value which is blank.

Essentially I only want the calculations of Unit Price x (multiplied by) QTY to show up in total collum only if they are used.

I'm assuming it may be as simple as a code used in Text field properties / Options / Default value location (see attached).

Sorry I'm quite new to JS etc.

Any help would be amazing, (if it's at all possible)

Thanks in advance